Hi, Cammy! Welcome back to the boards; stay as long as you like!
I'm not particularly familiar with European currency or the exchange rates, but I would guess that Emerald is about $35, maybe $40 in America, so I'll base what I say off that approximate price.
The most important thing might be to ask how far you are in Sapphire. If you've already gone through the Elite Four and done a good bit of the contests, then you've probably got about as good of an idea as to what the game has to offer as you ever will. (Smaller things, like individual battles and completing the Pokedex, might still remain; these aren't really major gameplay points, though.)
Really, I wouldn't recommend buying Emerald unless you're sure that you'll get a decent amount of enjoyment out of the extra features. Sure, the selection of Pokemon is a bit different between Sapphire and Emerald (as well as FireRed), but you can complete your Pokedex without Emerald, I believe, so that's inconsequential.
And realistically, the only major change between the two gem games is the Battle Tents and Battle Frontier. So let's take a look at those.
First of all, the Battle Tents are mainly preparation for the Battle Frontier, like a trial run of sorts. You see, the Battle Frontier offers somewhat different ways of battling. In the Battle Tents, stationed all around the Hoenn region, you can test out some of the newer battle methods for small prizes (stuff like Potions). You get the prize if you complete the challenge, but the prizes are small, so it isn't too hard to get the hang of most of the different styles. At least, it isn't difficult to master everything well enough to blast through the tents.
The Battle Frontier is a different matter. This is by far the biggest challenge in Emerald. In fact, one of the biggest reasons to get Emerald is for this incredible challenge; I honestly don't think anyone on TPM has been able to complete the whole thing. When the best team-builders in the AG (Advanced Generation, where this thread is) forum say they haven't even come close, you know it's incredibly hard. Think of the Battle Tower in GSC. Yeah. That's the kind of difficulty I'm talking about.
Now, the Frontier is set up sort of like an amusement park for Pokemon trainers. The biggest things to think about, though, are the seven attractions where you'll be doing that tough battling. I'll outline each section for you.
Battle Dome - This attraction is run in the second-most straightforward and familiar style. Every time you challenge this area, the game sets up a tournament with fifteen computer opponents and you. Before that, however, you must pick three Pokemon to be on your team. (This is pretty standard for the Frontier. For each attraction from now on, unless I say differently, just assume that you pick three of your Pokemon to make a challenge.) Then the game sets up the tournament brackets. It's a single-elimination tournament; win your match and you're healed, lose once and you're done. Before each of your matches, you can do a bit of an analysis of your opponent (as well as other tournament participants, if you like). You can look at their three Pokemon, their battle style, any special emphasis toward any particular type of move, and how they've done against any previous opponents in the tournament. But before the match begins, you have choose two of your three Pokemon to actually take into battle. From there, the battle is held like any other battle, except that each of you only has two Pokemon to work with. Repeat this process until either you lose or you make your way through all four rounds of the tournament.
Battle Pike - This is a bit of a weird attraction in that it's extremely dependant on luck. This time, you take your team of three into a series of rooms. These rooms come in two varieties. The first gives you a choice of three corridors to enter; the second is the corridor you've chosen itself. The rooms alternate back and forth. Your goal is simply to survive through 14 rooms (I believe... someone correct me if I'm remembering the number wrong). Simple, right? The problem is that you don't have a clear idea what lies in each corridor until you actually choose one. Some may be easy to pass through, others may actually help you, and still others will really test your skills. Good possibilities range from simply walking through the corridor to getting some or all of your Pokemon fully healed. Bad possibilities can be a trainer battle (although sometimes your whole party will be fully healed if you make it through the match), a double battle, or having one or more of your Pokemon inflicted with a harmful status condition (Poison, Paralysis, etc). Each of the rooms with one of the three-way intersections will have a lady in it who you can talk to briefly. She will give you a vague clue about one of the paths, but even if you can decipher your hint (guides are helpful with this), the actual nature of the path she tells you about will be determined through probability. (Eg. A 75% chance of a trainer battle, a 25% chance of being healed. Something like that.) Clearly, it's all about luck here.
Battle Palace - For many, this is the most hated attraction of all. Here, you battle like you normally against seven consecutive trainers (or until you lose). You get to heal after each one, so you're on an even playing field with each opponent. But there's one catch: the only thing you're allowed to do is switch out Pokemon. Your Pokemon have to decide what moves to use for themselves! This is determined by the nature of the Pokemon (Hardy, Modest, etc.), and unfortunately, the nature of their battle style sometimes is different than the strategy their nature would normally make them good at using. Also, when a Pokemon gets low on health, its battle style will often change. This is a difficult and sometimes frustrating area, but that pretty much covers the essence of it. (If you want a more in-depth analysis of the effects of natures, there should be something detailing that on the first page of the AG forum.)
Battle Pyramid - This is somewhat straightforward. Basically, you have to fight your way through multiple levels of a pyramid, moving around as you normally do on the map. But before you start, your items are confiscated, and you're given a replacement pack for the duration of the challenge. (You might be able to put items in the pack you take with you, but if you lose, I'm pretty sure you lose those items, too. Not sure about this.) Also, Pokemon aren't allowed to hold items here, so you're going with just your three choices and nothing else. Throughout the Pyramid, you'll find wild Pokemon and trainers to weaken your Pokemon. You might also come across items scattered across the floor; those are precious and should not be wasted! You'll want to use them only when necessary. The area is set up like a maze, but it's also very dark. Flash won't help you here; the only way to light things up is to beat trainers, like in the Dewford Gym. Your goal is simply to make it through all the levels of the Pyramid (I'm not sure on the number here).
Battle Arena - Ever had someone judge your battling style rather than just whether you win or lose? Don't want to experience it? Too bad. This is what the Arena is all about. You simply fight seven consecutive trainers (again, you're healed after each one). You're not allowed to switch out Pokemon until it's KOed... or eliminated by a "decision". What's that, you ask? Well, the battle is divided into rounds. The first round begins when you and your opponent send out your first Pokemon. Each time a Pokemon is eliminated, that round ends and a new round starts when the new combatant(s) is/are sent out. But if a round lasts through three turns with neither Pokemon KOed, the game will judge which Pokemon wins. There are three categories: Body (how much HP each Pokemon has remaining as a percentage of its max HP), Mind (the percentage of attacks out of the overall moves you used), and Skill (how well your Pokemon attacked; super effective moves are good, ineffective moves and misses are bad). You're given 2 points for a win in each category, 1 for a tie, and 0 if your opponent wins the category. Whoever has fewer points is automatically eliminated; in the event of a tie, both Pokemon are eliminated. This continues, round by round, and battle by battle.
Battle Tower - This is very simple. You just fight seven consecutive battles with regular rules in the three-on-three format. No gimmicks, no tricks. Just straight up battling. This should be very easy to get the hang of.
Battle Factory - Okay, now things get tricky. This is one of the weirdest attractions here, in that you do not use your own Pokemon. Yeah, you heard me. Leave the team behind, because you're going to have to take what you can get from the Factory itself!
This one is going to take a bit of explaining. At the start, you're given a series of Pokemon. With each one, you can look at its nature, item (if it has one), stats, abilities, type(s), and moves. You have to select three Pokemon, put them in an order of your choosing, and move with your makeshift team into battle. This is difficult, because sometimes its hard to know how well a Pokemon will do until its actually in battle. And other times you'll end up with a good portion of your choices being obviously useless, leaving you with a bit of an uphill battle, so to speak.
From here, you move into the battles. Now, the battle itself is done normally, like a regular three-on-three match. But afterwards, if you win, you'll have the option of swapping Pokemon with the trainer you beat. That is, you can (if you choose) take one of your three Pokemon and trade it with one of your opponents. But there's a catch. When you first chose your team, you got to see everything about each Pokemon you considered. When you swap with your opponent, you only get the names of the opponent's Pokemon. Any other information you get has to come from your knowledge of what happened in the battle. So you have to be really careful with this. Also, if you swap, the new Pokemon takes the same place in the order as the one you're giving up. So if you, for instance, drop the last member of your team for a Baton Passing Ninjask... you'd better hope you only need those first two Pokemon. It's tricky business.
Now, each attraction also has a leader who presides over the event. If you beat an attraction a certain number of times consecutively (yes, consecutively), the leader will appear and battle you under the rules for the attraction at the end of the last successful challenge you make. For instance, you have to beat the Battle Pike twice in a row in order to face the leader there (I won't spoil it for you). But you have to face her in exactly the condition you're in when you complete the challenge - like the rest of your journey through the Pike, you're not healed unless you pick a lucky corridor! Similarly, you can face the leader of the Dome if you beat it three straight times.
If you beat a leader of an attraction like this, you get the Silver Symbol for that area. Lose, and the number of consecutive wins resets to zero. But wait! There's more! If you can beat that attraction a greater number of consecutive times, the leader will challenge you to a rematch. The leaders will be harder then, though. Beat them again, and you'll get the Gold Symbol for the attraction. But it's not so easy to get to them a second time. (Well, it's not really easy the first time, but you have to jump through hoops to pull it off again.) For instance, to get a rematch in the Dome, the number of consecutive wins necessary increases from 3 to 6. The Pike becomes insane with this; it skyrockets from 2 to 10! Other attractions also have tougher scores for you to attain to reach the leaders.
Well, that's pretty much a rundown of the Battle Frontier, which really covers the main difference between Emerald and Ruby/Sapphire. If you have any more questions, feel free to ask. Anyway, see you around!