Assassin's Creed gets $32 and change to $36 and change at FYE, depending on which figure you go by that's listed on secondspin.com, since for some reason the PS3 and 360 versions are listed twice with different trade values(one is @ $26.75, the other is $28 and change).
FYE gives you 20 or 25% more for taking credit at their stores rather than cash, which is where I got those original values of $32-36 from.
Avoid Gamestop like the plague if you can, since if you go by the values people have mentioned already, the game you just bought lost about half its value already. That's worse than buying a new car.
Just trade it here or sell it to FYE, which wil get you alot more than Gamestop usually.