Colleges charge tuition; some more than others. Tuition is usually charged with at least some reference to the classes a student takes; thus a part-time student would pay by the credit-hour, and the full-time student pays a certain amount in order to take a full load of classes, whatever that may be for the institution. Tuition - the money paid - covers a whole lot more than the classes, of course: the buildings, the heat and electricity, maintenance and the people to do the maintenance, including janitors and grounds keepers; the administration and the administrative support, which includes the admissions office, the registrar, the bursar, and student services; of course the library and the books and staff; and other things a college student might enjoy, including athletics and those facilities - which were an integral part of the Academy - and probably more things I haven't thought of yet.
Tuition pays for all of these things; but if we think of there being a tuition for each class - as the part-time student pays - then we see that these costs are distributed among all the classes: for instance, every class in the philosophy department has a certain amount of tuition revenue associated with it, and one might reasonably assume that the support staff and office supplies comes straight out of that part of the budget (I don't really care if this isn't how things are actually divided up: they might be, but I'm speaking conceptually here). And all of the Liberal Arts classes can be understood as paying for that college's staff. And all the courses offered pay into the common features, such as athletics and the library. These are shared resources, thus the money generated from every class contributes.
Of course there's another wrinkle, which is that few students (or their parents) write a check for the full amount. Most have loans, many have some sort of scholarships and grants. So, the figures are squishy: I know that. I haven't seen the actual figures, and in some respect they don't really matter. The point here is, for every class, you could take the tuition associated with the enrollment and come up with a figure of what that particular class generates. If tuition for the semester is $10,000, and a full load is 4 classes, then each student is paying roughly $2,500 per class. (If you're wondering, I was inspired to do the math by this cartoon.) I think this is a legitimate way of looking at costs, because if the student wasn't taking any courses, then she or he would not be a student: there's no particular reason for that person to be on campus.
To explicitly connect Plato's Academy, the purpose of college is the instruction and learning that goes on; you don't need the admissions office or the library, or the copy machines and overhead projectors. You just need a quiet place to discuss issues, and someone who knows what they're talking about. I'm not suggesting that we get rid of all the non-teaching elements, but I do think it's useful to focus in on the instruction, because if you take away the instructor, nothing else makes any sense. So: what percentage of the revenue generated by each class ought to go to the instructor? This is not a rhetorical question.