for the majority who do it for income reasons, ideally it would be designed to not take you more than say, 8 years to pay off, given the median income for the major and area.
this alone would resolve most of the issues. there is value in someone majoring in English. issue is the median English major, given their inclinations are unlikely to make as much as the median, say, Economics major, in a given area.
these disparities would ideally and should be reflected in the cost.
the result would be:
1. when you apply to college you should apply to a major and that should be part of your application
2. different majors should cost different amounts of money
3. acceptance rates should differ based on major
4. classes of different types should cost different amounts of money beyond some gratuity (say, 8 classes).