This has been my experience too - I cannot remember the last time I saw a Mk III (no star) with the cut-off AND the volley sights AND the windage adjustable rear sights for sale. Like yourself, I have no doubt there are plenty of them in private collections, but they don't seem to leave those collections often. I asked around recently looking for one, without any luck at all - a surprising number of people didn't know they even made .303s with volley sights or windage adjustable sights. I'm told the Mk IIIs come up for sale sometimes at the twice-yearly big arms auction in Melbourne, but the prices from that frequently seem waaaaaaay out of my budget.
But isn't that the point. They are priced according to availability.