I traded in some games a while ago(Crackdown, Assassin's Creed, Rock Band, Gun(360), TH Project 8, Skate., Guitar Hero 3, Quake 4, and Full Auto) and had enough to get Halo Wars as well as some left over, so it depends what you have and really don't want.
Most of those games I don't play because of sequels or general boredom and was never going to play them any time soon.
Plus if you're over 18 you can trade them in for cash.