How much is a used 3g iphone worth?

I'm looking to buy a used 3g iphone that is 12-18 months old, what price should i expect to pay?

I pad 150 for mine from gamestop I'm actually selling it back to them cause the 3g I'd kinda slow and you can get much games on it anymore because they all need like ios or need to have faster proseesor to handle the big games I will sellmine to you or like 80 dollars it is in good shape no screen cracks or anything and I can unlock it if you want and jailbreak

$50 tops.
gamestop has 8gb 3gs iphone for $130

$ 150

But what if I wanted one for $100 and there's nothing wrong with it