How much do you think I could sell a T-Mo Galaxy S 4G?

I want to get rid of this phone because it's feeling its age (freezes on occasion, forcing a reboot) and I'd like to buy an unlocked GSM Galaxy Nexus.

Because my family owes me quite a bit of money, I just asked to be on their family plan to save the $40/month, so I'm not looking to switch carriers for at least a year.

My question is, if my phone shows some wear on the sides (the cheap chrome rim paint has chipped), but otherwise works fine (minus the freezes that might be fixed with a full reset), how much do you think I could get out of it? is only offering me $70. Any suggestions are welcomed. Thanks.