I did an informal post-bac at UC San Diego. They labelled me as a non-degree graduate student, so I paid graduate student fees (roughly $2,300 per quarter), and could take whatever classes I wanted. Being a grad student I got priority registration, and was able to get every class I needed and thus finish all of the med school pre-reqs in 1 year. I went roughly $20,000 in debt (subsidized and unsubsidized Stafford, plus private loan), but that covered tuition, books, and all living expenses for the whole year.
Since it was an informal program, they didn't hold my hand, or offer any guidance, so it's not perfect for everyone. You do have access to the undergrad pre-med advisors, though, so that was enough for me.