If you are at all interested in collecting arcade games, or just want to make a little money, then I would suggest picking up one of these machines right away. Let me tell you why.
Street Fighter 2 started the whole fighting game phenomena, it was a wonderful game, and spawned more clones than you can possibly ever count. It was so popular that Capcom was able to remix it several times, and sell it again and again.
This title had an absolutely huge production run, by far the largest production run since Pac-Man and Ms. Pac-Man were released in the early 1980s. Luckily in the arcade world, the more common titles usually become valuable over time, simply because of popularity.
This game is destined to be a classic, and right now these machines are at an all time low price. I have owned two of them, and I didn't pay more than $75 for either one of them.
The popularity alone is enough to ensure that this will be a valuable title in another ten years (Pac-Man machines were dirt cheap ten years ago), but there is another factor at work here that is going to make these machines more valuable than Pac-Man machines will ever be. You see the entire Street Fighter series were equipped with suicide chips. All of them. These games stop working when a little battery on their JAMMA board runs out. A fix has been developed, but no one is using the fix, because it is currently cheaper to simply get another JAMMA board that hasn't died yet. The thing is, you have to fix the boards before they die, and no one is really doing this. These machines are going to stop working in massive numbers in the next few years.
When something is both popular and rare, it is going to rise dramatically in price, and working Street Fighter 2 machines are going to start becoming rare very quickly. All you need to do to save one of these machines is to get the suicide battery fix done on it before the battery dies. There are several different fixes detailed on the web, and if you are unsure of your own skills, you can always send your board off to someone else to get it modified. It will be worth it.
If you are buying a machine. I suggest looking for a dedicated "Champion Edition" machine, as that will be the one to have in the future. The best place to find one is not on eBay, but at an amusement industry auction. These games go for dirt cheap at the auctions, you probably be able to pick up one of them for under $100.
While you are waiting for your game to become valuable, why not set it up to use quarters, and put it in a prime spot in your house. You will probably end up getting your initial investment back just from the spare change you toss into it.