The plastics cost are relatively low for this product compared to many other products made of plastic. Lego is first and foremost a marketing machine (which they excel at) before they are an injection molding specialist (which they also excel at).

They've had their ups and downs over the years but the value add on 1 KG of ABS to sell for >> $200 is such that the plastic bit is almost an afterthought. Without the StarWars franchise and discovering adults rather than kids as a market Lego would be in an entirely different position today.

Did they discover adults or did those adults just grow up with Lego toys and never stop loving them?

Most people from the Lego scene were out of it for a decade or more and then sort of rediscovered it, usually in tandem with their own kids.

I'd bet a large sum that the price of licensing the Star Wars brand is a greater part of the cost of a Lego Star Wars set than all of the plastic in it.

I definitely would not be taking the other side of that bet, you can simply compare the per kg price of the branded sets vs the ones that are 'just' Lego.