Walking around in an Australian bookstore at least I am still a bit flabbergasted by how everything is printed to be huge, everything a slightly different size, lots of paperbacks with glossy covers etc.
Not that I think this is a "cost of materials" thing in itself. But it all compounds on itself to where now a bookstore is huge to have just some random nonsense, and people will probably buy 2 instead of 3 books.
I agree that books are probably not "too expensive", I just wish that the mass market paperbacks would be smaller more straightforward and less of a precious little item.
To anyone interested in this stuff and in Tokyo(... well, Saitama), the Kadokawa Culture Museum [0] is ... probably the biggest building commemorating a publishing house in the world? The pictures don't do it justice, the building is ginormous.
But in it there's a bit of a (corporate approved) history of Kadokawa built into the museum. Their core thing that found them success: standardising a small pocketbook format for printing their books, having almost everything print to that size, with the same font etc, and selling it at a low enough price that college students could buy more books than they could ever read.
Printing all your cheap stuff in A6 sizes mean you can have a _loooot_ of books at home before worrying about much.