It's been a while since I played this game. Maybe a little bit more than a year, so maybe my memory is wrong and I got MW3 confused with another game.
But I remember there was some sort of way to get the DLCs for free or at least enable them to use while you play online. I think it was some sort of glitch. I believe I did it for MW3 and didn't have to pay $15 for each of the 4 DLCs of this game.