I almost never buy dlc - sp or mp. I assume that mp dlc tends to sell better than sp (for games that have it) because they are always immediately accessible - you don't have to have completed the game for it to be relevant - and they actually grant you something tangible. If it's a new outfit, you can actually go show it off in mp, and if you are perhaps in the younger generation and care about such things, seeing a bunch of people running around with cool stuff you don't have would be a major incentive.
But this just seems really wrong to me on so many levels. A Tomb Raider game with a bunch of mp expansion content? WTF?