Here's what the $10 online codes will do:
1. Due to the online code, Gamestop will have to sell the used game for $45, instead of $55.
2. This in turn, will force Gamestop to give less trade-in credit to the customer who's trading in that game.
3. This will make the customer less likely to trade in his copy, which he bought new.
4. This will make the customer less likely to buy the game new, since he knows he won't be able to get good money for it.
5. Thus, new game sales will decrease, possibly even more than the number of $10 online code purchases.
If they want to go this route, they should sell the game disk package for $50, and online codes, irregardless of whether the game disc was bought used or new, would be bought directly from the publisher's site and tied to a person's account. You don't violate first sale doctrine in that way, gamestop will have business as usual, since you're selling online portion as a service. You can even go further and make the online portion subscription based instead of 1-time unlock for extremely popular shooters like COD.
I'm sure I'm not the first person who came up with this idea, so no one's implementing it yet, it means the finances don't work out yet and online gaming is not as big as we would like to believe.