I work in a sixth form college. We get some where between £4500 and £5000 per student. For that they get something like 18 hours of contact time in groups of about 20 for about 35 weeks a year.
How a university gets to charge double for an arts degree seems to smack of inefficiency. Certainly buildings are used poorly
They also charge £9k for a chemistry degree / engineering degree / medicine degree, where masses of equipment and things are needed. Some degrees are profitable, but they subsidise the others that aren't. If you didn't charge more than cost for some degrees, then logically you'd also have to stop teaching non-profitable ones (chemistry being the obvious one that is always talked about). Which they mostly don't.
The whole system is also blooming confusing, because of the fact that universities are not primarily teaching institutions, they do teaching and research. They aren't glorified schools - in some departments teaching subsidises research, in others research funding means that expensive equipment can be bought that undergrads can also get to play with. If you understand that universities don't just teach and that the two strands are highly inter-related, a lot of stuff about them becomes a lot less obvious than it might at first seem.
And as for buildings being used poorly - if you've ever tried to get a room booking for a lecture theatre during the day in term time, you'd not think that buildings were being under-utilised. That sort of blatantly obvious idea has occurred to people in university administration also.
Because universities exist to make money and their reputations are based on exclusivity. You don't have the wrong end of the stick, that is held firmly by the collective hand of the UK's university Chancellors.
Only one (I think) university in the UK is profit making. The rest exist to do research and to teach students. They want to attract as much student money, because it can fund more research, and better teaching, but they aren't trying to make tons of money for itself.
(and before you ask, no, I don't have massive long summer holidays - that is when a lot of the research happens, and actually this year I don't currently teach at all)