First it isn't easy finding just a set of wheels. They almost all have worn out rubber on them but its expensive to dispose of so most ppl insist on leaving it on.
A good set of wheels in the classified section on tt.net goes for about $400+. On ebay I have seen sets go for about $250. I found a set in very good shape with a bit of curb rash for ~300 from a reliable seller ( not ebay ) and these are the set I decided to buy.