The reason Hex receivers are more expensive is due to advertising. The Hex rifles are older, but I think just as common as the post-1936 round receivers. Look at it this way, Russia had an army and navy exceeding 5 million men just about each and every year of the M91 and M91/30 issue, and a lot more in WWI, WWII, and the early years of the Bolshevik regime when the proletariat needed full employment or goodbye, Lenin. So there are literally millions of them around.
That being said, no reason not to pick a couple of them up, they have good stampings and lots of variations and it's only a few bucks more. $20 additional is fair, more than that is gouging, unless it's a really prime example.
Chances of getting something besides Izhevsk arsenal increase dramatically with the Hex rifles.