This was particularly problematic because my dad (a military doctor) had only been out of the military for a couple of years. Meaning the salary he we were being pegged at for financial aid was something he had only been earning for 2 years or so. He mad significantly less in the military.
Point being: For some folks, in some situations.. the cost is well beyond reach.
EDIT: Hell, look at the comment below; the net price calculator (http://npc.fas.harvard.edu/) even has a drop-down for number of children in college.
The issue with CMU is that, essentially, they don't have much money. Their endowment is much smaller than most of the schools they compete with. In terms of the "top" CS schools, they're probably a bit of an outlier.
That's one of the reasons I ended up not going to CMU. Well, that and the fact that I didn't want to trade the Bay Area for Pittsburgh :P.