First off, the trade-in values aren't universal to all stores. Cheapassgamer said I'd get 25 for Portal 2, and I got 16. And as for the Skyrim thing, if it qualifies, you need to ask for the deal in question AND use the entire stock of credit towards the preorder.
the girl at the gamestop that is close to me told me that you only get the extra 30 percent for NEW preorders. i already had a existing pre order for skyrim so she told me the extra 30 percent didnt apply to my trade in. i know i could have simply cancelled the pre order and then re did it all but instead i just took my business elsewhere.
"I consider myself an artist, but I would give it all up for a boatload of money" Jim Gaffigan