okay if it's a newer 360/PS3 game expect somewhere aroudn $27
if it's a newer Wii game expect somewhere around 20 (although i got 27 for metroid a month or two back)
if it's an older PS2 game, theres really no way to tell. anywhere from .50 cents to 15 bucks.
i usually trade in to gamestop, i like gamestop, they treat me good there. though they may rip me off on trade-ins the edge card really saves you money when trading in/buying used games so it really balances out in the end.
if you have 10 games to trade in for an older system expect somewhere around 60 buck for them wich isnt half bad.