labor cost is the most expensive part of a $300-500 knife...the steel cost is abotu $10 usd.
its faster and less wear on the belts and stones to thing/grind on un-hardened iron or mild steel,
especially during finish sharpening/shaping, saving alot of labour and consumable abrasives
not to mention simply using less production time so faster overall.
Monosteel is still used in honyaki, but other than that, it's limited to certain production environments--
primarily those that are automated at least in part.
That being said, san-mai is no longer any gauranteel that the knife is substantially "hand-forged",
even expensive knives in $500 price range are using factory billets/clad, and I'm guessing anything
using PM steels and most if not all more plain-vanilla stainless is also pre-fab bar stock.
Its very difficult to get good weld on the cladding with stainless core when doing it by hand,
in open air, etc ... and needs alot more than general skill with blacksmith tools.