How much is a used iPhone 4s worth?

I just got a new phone, and now I'm going to sell my used iphone 4s… Its 16GB and on the back it has a ton of cracks. It works fine though… Could i get more than $50? Also its AT&T

Look on ebay for past sales. Then figure in selling costs… About 10%+3% paypal fees.+ shipping.

Its worth something.