What is the Used Value for a Good Condition Apple iPhone 4s 16GB?

I want to sell used cellphones to craigslist. First I must know what they're used value are.It'd be nice to find a website that tells you the value of a used Cell phone kind of like Kelly Blue Book for cars. I did find this website http://www.tagtotrade.com/, but it doesn't show the value for an iPhone 4s.

400 - 500 should be a good price. Just sell it before the i5 comes out.