Re: Aliens: Colonial Marines.
Trade in value right now is $20 without any of the stackable bonuses.
So 25% bonus brings it to $25 then 10% more for trading in two games brings it to $27.50 and 10% for PUR brings it to $30.25 credit towards Bioshock.
Correct me if I'm wrong. I don't trade in games but I am on CAG a lot.
The only thing that might not work is doing the preorder bonus with the tier trade bonus. But very YMMV