I've also heard the collapse of the rack trade being used as an explanation for the "extra 300 pages of crap" school of writing. Apparently, the majority of paperback sales in the US used to come from the spinning racks in supermarkets and general stores, where a very thick book could take up the space of several thinner books without a corresponding increase in profit. In those days, US publishers actually discouraged overlong books, unless they were by known bestsellers such as Stephen King and James Clavell.

The rise of Borders and B&N, which had enough shelf space to display paperbacks as thick as bricks, meant that this was no longer an issue.

OTOH, US paperback prices don't seem to have risen as drastically as those in the UK and here. As best I can tell, looking through my library, they've roughly trebled over the past 20 years, while prices here have increased by a factor of six or more. Here, of course, we're supposedly paying a premium to keep Australian publishers (mostly subsidiaries of British publishers) afloat so they can publish Australian authors. In reality, Australian publishers are so contemptuous of the backlist (anything more than a year old) books by Australian authors are more likely to remain in print outside Australia, and schools and universities have to import them!