>>424676103rd versions always sell relatively poorly because of the additional big investment from many people, so many choose simply to buy the first game. in addition, some people skip the first version, and if you're buying the 3rd game first, you have no reason to go back and buy the inferior first version. thus, in the end, they're getting just one purchase from a really significant portion of people.
DLC, on the other hand, is a lower monetary investment from the player, and as there's already an established "usually you pay $120 for the first game and the 3rd version" (which is bullshit if you aren't an impulse buyer), people will think it's a good deal and will be more likely to get it. in addition, people who would normally skip for the 3rd version would only spend $60, as they'd have no reason to go back and buy the base game. however, because of the current setup, all of those people now have to buy the base game anyway, and thus have reason to buy the DLC.
if you're too retarded to read all that, the short of it is that yes, DLC is cheaper, but it also sells more.
