I do think there's an argument to made that college should be cheaper (at least excluding housing and other secondary costs) than K-12 given that K-12 has a bunch of mandatory cost centers like special education (which is to say dealing with kids who destroy classrooms, abusive parents, etc.), that class size can be larger in college, that students in college (at least excluding community college) should be above the average in K-12, time in classroom per student/semester is lower, etc.

But the salary of the faculty is more expensive, at least on a per instructor basis. Although maybe you need fewer instructors. And facilities will cost more. You're not likely to have a nano-technology lab at your K-12 school.

I think university could be cheaper than it is today, but probably not much cheaper than K-12.

Frankly education is too cheap, which is why people are getting crappy outcomes and then debating on it.

The idealized form of studying is education with mentoring, see Blooms 2 sigma results.

But that is too expensive, who wants to have that number of teachers ! It would cost a fortune!

So instead people try to cut costs, and or keep making it a discussion on price, since this is America, instead of just making colleges public and free.

Of course actual faculty wages are balanced against boatloads of adjuncts and grad students working for a pittance.