Dear Cal,
Fair enough: 50% is not trivial. But might it not be the case that the price before was ridiculously low, and is now more realistic? Not knowing about Mamiya 6 cameras, I can't really judge. What percentage of the new price are they selling for?
Perhaps we have been used, for too long, to the aftermath of the Gadarene rush to digital. People bought and sold good cameras because they were dirt cheap and it was almost a pocket money hobby. Now they're no longer dirt cheap, and a few digi-converts are buying back into film, they look expensive.
I don't know. But I am reasonably confident that compared with what second-hand top-flight cameras cost 20 or 30 years ago, there is a much smaller price difference between interesting but non-pro gear (Retinas and Olympus OMs, for example) and top-flight cameras such as Hasselblads and Nikons. They tend to be lumped together now as 'film cameras' for 'enthusiasts' or 'collectors' ('collectors' in this case including people who put a few rolls of film through their cameras per year). Formerly cheap cameras cost proportionately more; formerly 'professional' cameras cost proportionately less.
Cheers,
R.