AT&T's website lists the white iPhone 3GS (32gb model) for $149, refurbished. I'm not sure they actually still produce the 32gb models anymore since the release of the iPhone 4. After looking on Apple's website I was only able to view the black 8gb iPhone 3GS.
There around 300 on ebay but if you don't have a plan get an iPhone 4 because it would be cheaper
well not cheaper just more worth it