Those RSM Rugers are absolutely gorgeous rifles...but IMHO they are just way too heavy for a .375H&H. I have had one in .416Rigby and it was a beauty...up until the stock split at the tang just a few weeks before my scheduled once-in-a-lifetime hunt in Africa. I was shooting it...a lot...leading up to that trip. I was bearing in mind what several of the old-time gunwriters used to say, regarding the idea that big boomers were not intended for extensive shooting, but that the .416 would be fine. When I saw that crack appear, the air was blue...for days...
But I had the chance to shoot an RSM in .375 and, as beautiful as it was with that integral rib and nice Circassian walnut, it was a pig to handle. Apparently they reduced the weight of these guns at some point during the production run? I don't know if the one I shot was before or after the reduction, but it was like swinging a lead pipe around. Just too much weight; a disproportionate amount at the muzzle, but honestly just too plain heavy all around. The .375...despite the usual wails from those who have never had one...is not a vicious kicker that requires such a heavy rifle.
I wasn't able to compare the two rifles directly. Did Ruger use the same exterior barrel profile on all the chamberings, making the .375 the heaviest of them? Sure felt heavier...
I've seen a standard model 77 that had a minimal amount of metalwork done to it to allow the chambering of .375H&H. No idea how it shot, but it felt nice, much preferable to (but not, of course, nearly as ###y as...) the RSM.
And, of course, we periodically see standard 77's on the EE chambered to .458WinMag. The sellers always seem to think that the chambering somehow magically morphs them into the much more expensive RSM model, so they price them accordingly.
The barrel profiles are the same for all of the RSM rifles, hence the .375 is the heaviest and obviously that is primarily toward the muzzle... while I dislike the muzzle heavy feeling, it is very stable to shoot off-hand. My RSM in .458 Lott on the other hand is wonderfully balanced. IMO all RSM's should be bedded. As for the M77 Tang .458 WM, they have a reputation for being accurate and reliable rifles, and all of the online talk regarding these factors have caused the prices to creep upward over the past couple decades. However, as you say, it is insane for any seller to believe that the standard "R" model M77 is worth MORE than the RSM. My current RSM .375 H&H is my fourth, so I am very familiar with it's characteristics... the only change to the weight from the original run to the final run was the position and design of the front sling attachment from a classic barrel band to the "through stock" forend attachment... I doubt that anyone can tell any difference in weight, even on a scale as the wood density would be a greater factor from rifle to rifle.