You should be able to find a working Bronica ETR (or ETRS or ETRSi) with a prism, 75mm lens, grip and 120 back for under $400. They currently seem to be about the cheapest medium format system SLR camera out there. They may not be everyone’s cup of tea, but they are very capable cameras, and lenses are cheap—most are $100-150–so you can build out a respectable system inexpensively. You can also find older Mamiya 645 cameras for a similar price, but they seem to have creeped up a little more. They seem to be great cameras, but I have no experience with them.
for cheaper than that, there is a long history of folders from dozens of companies, and some inexpensive TLRs. They are going to be less flexible with fixed lenses and many are old enough that they won’t feel like a modern camera. I own a couple old folders, and personally, I wouldn’t recommend them to a beginner. They are just too finicky, IMO. (Maybe not to shoot, if you have a good, CLA’d model, but buying them have a lot of pitfalls.). TLRs feel more usable than folders to me, but personally, I’d stick to ones with automatic film spacing, so you don’t have to worry about red windows fogging you film (or the numbering being too light to see..