I wonder how much of the cost of books is because of physical production, and how much is because of the work put into making the content, as well as branding/IP "premiums". You could argue that with cheap printing done in China, that it only costs.. say.. well heck I dunno, $4 to print a book the size of the 5E PHB? Maybe less? I'm not entirely sure. You could argue that books that print as many copies as the 5E PHB have printed, the "real" cost of even the printed book could be 1/3 as much and still be "profitable" based solely on the printing cost.
Of course I also wonder how many books are set at certain prices because that is what the "expected normal" price of the book is for that industry/genre/tier, and how many books are actually priced based on the cost going into them for production as well as the printing costs, and then some margin of profit factored in. I don't know. It's something to think about.
I know a pdf version of a hardcover/softcover book still has all the writing/editing work going into it, the same IP costs or whatnot, and the only difference then being the printing cost.