I justify a purchase based on utility. If I can afford it (BroadSTPhilly is right, that's the first question you need to ask), then the question is: do I really need it? Can what I have already do the job, and if not, is this something I really need to do, or want to do, and will do often enough that I'll get good use out of it?

Mind you, I speak as the owner of four bikes, and I definitely don't get full utility out of them. One, however, is a hand-me-down that I just got from my brother, which is still disassembled. Once assembled, it will probably become my road bike if it works out for me (and my current roadie will get sold) -- if not, I'll be stuck trying to think of something useful and tactful to do with this bike. Another one is a discarded mountain bike that I'm fixing up and improving my mechanic skills as I do so -- once I've got it fixed up, I'm going to give it away. So that'll have me down to one roadie and one commuter.