Probably the cheapest MFDB for unfettered, untethered use (i.e. shoots to CF card, has a screen to evaluate results, and compact detachable Li-ion batteries) is the old Kodak Pro Back 645M. This won't fit a Hasselblad V model, but you also mentioned you are looking for a MFDB for Mamiya; it fits the Mamiya 645AF[D] line. There were also variants for the Contax 645 and Hasselblad H1. And BTW it also shoots tethered over Firewire if you so wish, or with alternative power sources (AC mains transformer or Quantum Turbo battery).
Anyways, I have one and love it. Got it, just-serviced and low-mileage, for $2700; and they've only come down further in price since then. It's still supported by Midwest Camera Repair, who bought all Kodak's stock of spare parts - and who resell/service them very affordably in comparison to the other MFDB manufacturers. For example, I recently got a
brand new replacement IR filter and 2
brand new batteries, for the princely total of €210 (no, there's no missing zero there!). Try doing that with PhaseOne, Leaf or Hasselblad!
BTW, the clip-on, clip-off IR filter makes this a fantastic back for switching between regular and Infrared photography, if that's your thing.