i would think five to ten percent of their income would be sufficient, per year, for ten years. no loans, just payments in the future. the government can give schools a net present value of estimated future payments, and collect the payments on income taxes.
this would incentavize colleges to make their students economically viable grads, which is what the end goal is anyway. they would focus more on practical skills. they might decide that four years and excessive unneeded classes aren't necessary, making it more efficient again. (though the government can require some very basic courses, like generic psychology and generic sociology and generic science etc etc and basic math and reading and writing) this also requires that students dont just get a free ride, but that they chip in on their own education, which would appeal to conservatives. students with not much intelligence or potential would be saw for who they are, and they would be found to be maximized to their potential by the school. majors that are worthless wont get as much money, and that would cause the system to adapt... maybe only the cream of the crop students should be doing humanities, and their would be a punishment of less money to both the student and school for allowing low skill students to go into the humanities. maybe a philosophy major will end up at mcdonalds, and neither he nor the school will benefit much. maybe the school wont accept stupid kids in the humanities.
the well off grads would pay more, who are the most economically viable ones. and the less economically viable ones would pay less. there's a certain justice to that, if that happens to fit your political ideology.
this proposal is an example of the kind of concrete solutions that politicitians should be working on. the beurocrats in washington have lost the policy in politics.