Is a iphone 3gs 16gb with a defender otterbox worth $100?

My aunt is upgrading her phone to the iphone 4s and told me i could have her current phone for $100. It is an black iphone 3gs with 16gb memory and a defender series otterbox. Im pretty sure that the otterbox has always been on there. I currently have a contract with a samsung captivate 16gb. If i put my sim card into her iphone 3gs, i wont have to aait ti get a "new" phone.

Does this sound like a good deal to you guys?