You are better of selling it on eBay. None of the trade-ins at Gamestop are worth it. eBay is great because you can sell it for more than GameStop will give you, but less than anybody else can buy it anywhere else. I would imagine you could get at least 90 bucks on eBay, especially if you set it at a starting price of $70 bucks and it is in good condition.
For the most part, though, none of GameStop's trade-ins are actually worth it.
I would try Amazon also.
If you aren't willing to go through the trouble of listing on eBay or Amazon, then $70 bucks may be your best deal.