660 and 660Ti will have to be around $50 tops. You can get a brand new GTX1050 and get roughly the same performance at $110
Possibly $75-100 for a GTX960. You can get a new GTX1050ti to replace that for $140
GTX980 is a tough sell, maybe $200, though I tried and failed to sell some reference 980s at that price. A GTX1060 6GB is almost as fast and has a little more memory at $240.