I recall something I read ages ago from 80,s/90,s last Century.
Airfix, Revell etc were starting to produce their kits using Copper Beryllium moulds to give a higher standard of detail.. i.e engraved panel lines which were difficult to do with tool steel and the spark erosion process. Unfortunately ,I think the use of Copper Beryllium was banned in Europe due to it,s high toxicity.
As I recall, production was then moved to the Far Eastern countries where I think Health and Safety Standards were lower
Another factor in the cost is that The Copper Beryllium moulds are made using stamping or pressing as the alloy is fairly "soft" compared to Tool Steel.
Being softer, they are not as durable and when the components are out of tolerance , the mould is scrapped, re-melted whatever and a new one made as they are not able to be re-machined. Expensive
So I may have been a bit harsh on the cost of plastic kits...Maybe a manufacturer would like to pitch in???