"Plastic frames generally cost less than $5.00 to make. They do not require expensive machining to final tolerances, either. So, they are much more profitable for the gun companies. That is MOST of the reason for their existence.
The all-metal guns which are still in production are increasingly being moved to off-shore production (to decrease labor costs). The remaining domestically-produced all-metal guns are becoming the top end of the market, price wise, at a rapid pace.
A thoroughly unfortunate trend, in my opinion....but no doubt inevitable, given the cost/ profit difference."
You seem to be forgetting the extensive design, testing, massive investment in tooling costs. I would imagine the tooling to make polymer framed items is a lot more custom than a mill and a lathe also.
Poly frame guns, for the most part, end up costing less than a metal frame gun for the end user, as well as manufacturer.