[quote name='gerst19']I have never heard of a company having 2 different prices for PS3 and the 360 for the exact same game. Just look at Target, Best Buy, Toys-R-Us, etc. But that's Gamestop for ya.[/QUOTE]
Those are new games prices that you are comparing to used game prices from GS. It is not quite the same thing.
They sell the new games at MSRP when they receive them from the publishers. However, used games are driven by the trade in market. Like an earlier poster pointed out, there are often more trade ins for the 360 version of a game for various reasons. Then, it is just simply a case of supply and demand.