Regarding welding bottles. I'd guess and this only a guess, that commercial gas bottles (of which I own ~6) are built heavier for long term service in demanding applications. Welding shops are typically rough places, construction sites are worse. I know my 244 cuft argon bottles are plenty heavy ~ 160 lbs.
Is a "lighter" weight scuba bottle more costly to make? Not sure but it's possible if methods and tolerances are closer.
I can say that all metals and particularly Stainless and mild steel have shot up in cost. SS is 2x currently from the price last September.
What is driving this? The war, the demand in China and the weakness of the US dollar.
I did have occasion to tour the Norris Industry plant here in LA a couple years ago before they closed. They made two things..... high pressure gas bottles and artillery shells. Same basic processes involved, deep drawing with jumbo presses.
Here again I'm speculating, but it would not surprise me to learn that both press capacity and the steel required is flowing to the war effort and not scuba.
Regards,
Tobin George