I had a look at my local Craiglist in CT today and you can pick up 3gs 16G as low as $170 and there is a 3GS 32G for $200. Some of the adds stipulate you can only collect on 25th June. It looks like most people are only looking for enough to pay for the upgrade to get the new Iphone 4.
I bought a 16GB 3GS with a couple cases and such for $250, but most people in my area are selling them higher because of the potential unlock for 4.0 General Release the dev team plans to debut.
It really depends on how quick you can find one and/or how desparate they are in selling.