A popular rule of thumb is to look for something made of Reynolds 531...that's a good tipoff to a pretty high-quality frame. Shouldn't have too much trouble finding something nice in your price range. I'm partial to 70s Raleigh Competitions and Internationals (they have Nervex lugs, the latter chromed), which show up often. A friend got a gorgeous Schwinn Paramount road frame that he's converting to fixed for under $500.