To find this out, go to Blockbuster.com and search for your game. The price listed will be what you owe, maybe minus the amount you already paid in rental fees.
It depends on the game, and how old it is. New games cost the most, but you might get a second hand one at a slightly marked down price, or a copy they have removed from rental circulation for even less... I don't know where you are, but being a Brit, the new games cost around £40 or so here, but the mark-downs vary wildly
