You can still sell your 3GS, and since it doesn't come tied to a contract (with a $300 ETF), you should be able to get a good amount for it.
The only problem is craigslist/ebay/etc. become absolutely flooded with previous gen iPhones right around the launch of a new one. People willing to go a bit lower on the price can get theirs sold faster.
I've had more success selling my old iPhones to people in person...that way they can see it and take delivery faster...I'd say at most you can get 200-300 for one that's locked to AT&T depending on the memory and maybe 100 more if it's unlocked...there will be a plethora of pre owned 3GS for sell this summer