Deep Astonishment by Paul Harris is one of the most killer effects you can do for anybody, and costs only $40. There's so much killer effects out there that are great bang for the buck, but it all depends on what you value as great and if that great effect is worth the listed price to you. That said I agree with _kfc on this one, and that is forget the best effect, and go for the knowledge. Get educated in magic, buy the books, learn about every type of magic there is, then buy the dvd's on the type of magic you want to learn (ex: Card magic, coin magic, rope magic, stage magic, close-up magic, ect.).
Example if you want to learn card magic, there's no better resource then Easy to master Card Miracles vol. 1-9, and you don't have to be rich to get them because you don't have to get them all at once. Just buy one dvd at a time, learn that, then save up and buy the next one, and so on till you have all 9 dvd's. The dvd's all cost around $30-35 bucks apiece and you get 10-12 tricks in each dvd which is great value for your money. And you can do this with any subject you like and want to learn. Just buy what you can, when you can afford to, and before you know it, you'll have enough material to last you years.