Unlike companies that care about their fanbase such as Bungie who make the Halo DLC maps available for free to all players after a few months, all Activision cares about is more money so you'll never see the DLC maps drop in price, even if the game is a decade old. At the least, games like Blops have offered bonuses like Zombies, or MW3's survival mode with Chaos added (not much of a bonus there though), or the new Extinction mode, in order to get players more interested in buying the DLC since there's more you're purchasing than just a few multiplayer maps.
You'd think they'd at least offer a game of the year edition or something with the DLC included, but those never come out because CoD is as disposable as modern day Top 40 pop music. Out with the old, in with the new, oh and the old extras aren't free even if they've stopped working on the game or even supporting it. Welcome to CoD, enjoy your stay with an empty wallet.