It says on the CAG home page that you get Killzone 2 for $52 shipped at amazon with the coupon code. When you enter the coupon code, it does take $5 off, but that brings it to $55. Where are they getting $52 from?
When you go to the home page, it should say that Amazon has recommendations for you. Click that link, then click the link that says see all recommendations. There's probably going to be a lot there so click on the items you own link. You're probably going to have a lot there, and at least something will be related to Killzone 2. Select Don't use this item for recommendations for every item but the one you think is related to Killzone 2. If you don't have any related items in there, search Amazon for Resistance 2 and pretend you already own it by writing a review for it. Now, go back to the recommendations page and select Not interested for every item but Killzone 2. The only recommendation you should have left will be Killzone 2. Now, wait a day, and check your Gold Box. Scroll down to Quick Picks and Killzone 2 should be there. It'll give you a discount of 5%, which'll save you about 3 bucks.
You can also manipulate this system for other games.