Depends on condition of device, battery, and any accessories.
May want browse the "for sale section here, Ebay and other such places to give you a good idea of going prices. Your carrier may also give you a credit for it if you are looking to upgrade and stay with them.
The BST Thread in the US Cellular section of HowardForums is a good place to post as well. There are more US Cellular users over there, you should get better traffic.