I wait for the DLC to go on sale at Steam and then pick it all up for the price of one. That's how I did it with Borderlands and Civ V.
I'm not going to pay full price other wise.
edit: I should add that when I say I'm not going to pay full price, I mean I do not need it.
I almost solely buy my games based on MP gameplay. Do not care enough about SP, though Borderlands has changed my mind to a degree. If Steam wants to do a sale on it that makes it 3 for the price of 1, then I'll probably jump. Otherwise the motivation is not there for me to buy it. It's not really a morality view (in my opinion....yet), I just do not see the need when I'll either a) never use it or 2) it will go on sale anyway. (see what I did there?