Well, it's a bit more complex than that. Many are supported through federal grants, not state monies. Now, yes, those are still tax payer dollars. But federal grants are contracts to perform a service, much like building an f22 or whatever. A pure libertarian position would be that we shouldn't support research on a federal level. I actually don't agree with that as I think research is a critical component to driving quality of life and general development of humanity. Also, when supported by teaching assistantships, these graduate students are performing a necessary service for the university. This may be state dollars (if a public school), but again, they need people to teach classes. Often though, a minor amount of state dollars fuel psychology departments, which get revenue from many sources.
In the case of professional schools 90+% of their funding is coming from student loans. That's a very different thing.
The exorbitant cost is not independent from the quality. You think you are going to get good quality students with the financial parameters the way they are now at professional schools? I'd say no. You are going to get people that don't understand money. You are going to get people who don't have good connections to the field and don't understand the situation. Basically, we're going to prey on the uninformed, the naive, and the stupid. Doesn't sound ethical to me. It sounds like exploitation. It also doesn't seem like the best way to advance treatment for mental illness. Actually to me, it sounds like we're saying that mental illness is an unimportant problem. We don't need bright people in the field.