Well, it costs 10x more than 128x128 textures, and that is about 380k tc for a full set (I think). So, it will take you a while. I suggest not getting a 512 set, or at least not starting to think about it until you have more than 100k, or are willing to buy tc.
512 does make a huge difference but if you were to get a full set get a 512 head and 128 body textures. The leg textures dont really make a difference unless you have a set like toasts or urics.