How Much can I sell my 360 for?

I have 31 games ranging from 06-10. 2 white controllers. The 360 is white. I have a hard drive not for sure how many gigs maybe 8. I have original box and all wires except for headset. I'm not looking to sell it on tha internet. How much should I sell it to a potential buyer? And how much would gamestop give me?