Used LX rear derailers can be had for peanuts. That being said, almost any RD with enough capacity will do. (And, yes, a short cage will do as well if you're conscientious about gear combos, but I've always felt that one ought to set up a bike in such a way that, should you lend it to some sort of dunce, the RD won't tear off when they try to shift to big/big, and the chain won't drag the ground when they try to shift small/small....)