How much credit will gamestop/ eb games canada give me for these games?

It all depends on the supply and demand. While one store may offer $30 for both, a different store might offer $20 if they have more of them in stock and less people want it. I've tried to call to get quotes before, and Gamestop won't do that over the phone. So unfortunately, you'll probably have to bring them to the store to find out.