Depends which console they are for and how new they are. If you sell them a like-new game that just came out you will get around $45. If you are selling a like-new game that is from 2003, you may get $15 or so. Its all about supply and demand.
The other thing to remember is usually get store credit, not actual cash. So if you buy a game from them, it'll be at a reduced cost. But you can't get $$ for your games.