Duder, let me clue you in on something. You're going to get damn near nothing. I was in an EB that was selling Angel Season 3, a $50 DVD set, for $13.99 used. Think of how much credit they must have given for it, probably $7-8. The credit you get depends on the disc.
Now, Blockbuster's doing a great deal where they gurantee you $8 per DVD, at least, as long as it rings into their system, which is damn near anything (minus Music DVDs, bands and things like Happy Tree Friends or some obscure anime). If they have a GameRush, well, you can use that credit to buy games. If not, use it to buy DVDs instead.