(now the comment)
The most expensive, of course, is going to school and not finishing. If, say, 70% of marginal college students wouldn't graduate (currently half of students don't finish their program in 5 to 6 years-
http://nces.ed.gov/pubsearch/p... - and there's probably a selection effect that means the marginal students would do worse), how does that change the rate of return?"
From the failed students' perspective, it is a disaster. Had they taken a skilled trades approach, they would have a paying job and marketable skills and much less debt.
From a social perspective, it is a disaster. Lives delayed and capital sunk with zero return.
Most people do not need to go to college.
