I am almost sure that there is really between the trade in and actually buying a new game. As usual, Hasting's has taken the lazy way and just bumped the TIV of all the games to at least $20. Hey, I'm not complaining..as usual, I will take very liberal advantage of this promo without having to put the credit towards a new game (although I will of course be getting ODST at midnight anyway). I traded in a single Riddick last night at got the $20 price.
Hilariously enough, some of these games can be had AT HASTING'S for less than $20, especially if you are using one of their numerous 30% off or B2G1 used sales. I'm not going to do all of your homework for you, but just dig around the store...you might be surprised! Also, as mentioned before, a TON of these games can/could have been had from various clearance sales in the past few weeks (BBV, BB, GS, Target, etc...)
Good luck!