I was kicking around my local camera shop the other day, when I found a bunch of Mamiyas. Two RB67s: back, dark slide, WLVF and lens, both $100. There were also some 645s, old and new w/lenses for around $150. I didn't test them, but they seemed to be okay (shop said they were alright but as-is.)
Tempting, very tempting Even though I'd likely almost never use them ... $100 isn't that much money is it?
You need to do some research on the production dates of the Pro, Pro S, and Pro SD, along with the usual KEH/Big Auction Site visits. I've seen Pro kits like this one that were smoked from years of heavy studio use that weren't worth $100. Leaky bellows, shot light seals on the backs, iffy shutters, frame spacing/advance issues are common on elderly RBs.
One very nice thing about MF cameras like that is they are modular. If you have less-than-perfect module, you can just get another one and swap it out. So, it's not like all-or-nothing proposition.
You do see low-mileage Pro S and Pro SD bodies occasionally, most of them back-up cameras that saw no or little use. I got one last year, a NOS Pro S boxed with a new 120 back. Because the back and RB adapter needed new seals(a DIY no-brainer), no one wanted it and it went for $120. Lightly used RB 67 gear isn't that uncommon. Be patient and be willing to build up a camera bit by bit.
$100 is a great price as long as it is working. You will more than likely have to replace foam seals but that is a DIY weekend project that won't cost you more than $20.
The lens shutter should be checked before you leave the shop. Dry fire it off camera at all the speeds as you look through it to be sure the shutter is openeing n closing properly. If not, plan on having it serviced for under $100 or plan on buying another used lens for under $100.
The body should cock n fire normally with n without the lens.