On eBay, you could get about $150 for it if it's in great condition. If it has the original box + inserts, accessories, etc., about $10-$15 more. And if it's unlocked, you could probably get up to $200 for it.
I sold my white 32GB for $130 on these forums yesterday. I could have gotten a bit more on CL if I wanted to wait but I just wanted to sell it fast to buy my 4S.