How much can you trade in a iPhone 4g in at gamestop?

How much money can I get for that. It's really kind of new, doesn't have any network problems you can use it forever. I cracked my screen because it fell out my pocket but got a new one replaced so the phone looks brand new. How much do you think it'll be worth? I'm selling it because I have the 5 already and AT&T only would give me 40 dollars to my next bill. I think I can get more than that

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