Normally wouldn't shop at Gamestop, but they offer 30% off for trade-ins towards this game. It doesn't specify what you have to trade in though. I'm sure they wouldn't appreciate it if I gave them a bunch of shitty Gamecube games though.

Anyone have any idea what I'd have to trade in to get the discount?

As far as i know its all trade ins no matter what, maybe you can nickle and dime your way to about $20 with enough random games, i personally traded Armored Core 5 and Battlefield 3 for $24 towards credit for Halo 4. They clerk told me "were taking an extra 30% in on all trade ins toward halo 4" or something along those lines. I mean it wouldnt hurt to try and trade them in no matter what, all they can do is tell you no