If you're on a constrained budget, a 2500k is a very viable option. I have a 2600k and you only gain hyper-threading. Odds are, if you don't have any dedicated software that is multithreaded, you're wasting your money on a 2600k because you won't actually be using the extra features you're paying for.
You won't find an Ivy Bridge in your price range within the next 12 months that is comparable or better than a 2500k. Don't expect Sandy Bridge prices to drop though either, they came in cheap for a new release so they won't really go down much than what a good 'egg deal can get for you now.