This is a discussion on 3Gs Going Market Rate within the iPhone 3GS forums, part of the iPhone 3G/3GS Forum category; How much are 3Gs's going for on the market (averagly)...
But of course all depends on condition / memory size / unlocked and so on, the best method is to check ebay and tick the completed listings box to see what they went for
Does $175 seem to be a fair price for a 32GB 3GS with the case cracked about 1/2" down around the connector due to having a case on it while charging on a dock?