Mamiya (manual focus) and Bronica ETR series go the cheapest of the SLRs at this point, I think. I don't have much experience with either system, but I am sure both are great. One of the nice things about MF is that there is very little 'bad' equipment... much less consumer junk floating aruond than in 35mm, and cameras tend to have been kept and maintained well.
Pentax 645 have more demand and go for a little more, probably because the lenses are fully usable on a modern digital system. But also a nice system.
What interests you specifically about getting an SLR? RB67 is a very good pairing with a TLR - it's big and best on a tripod, but very flexible, with a huge negative, good glass, bellows for close focusing, extensive lens range... I went with a Hasselblad because I wanted high degrees of portability in my system and found a good price. Bronica SQ are a great equivalent for about half the cost of a Hasselblad system and very versatile.
While manual focus 645 systems are cheaper, I think it is worth the little price bump to go to 6x6 or 6x7 if you can save a little longer... If I was looking at a 645 system it would be for a modern, AF system (Hasselblad H, Contax 645, etc.) which are big $$$ but have other advantages.