It was at University (of Washington) Book Store, not the one near the campus but a branch well north of Seattle. They still had plenty left. Rather than rip the covers off and turn them back to the publisher (Harper) they put them on sale at real clearance pricing. It pays to nose around your local new/used bookstore and see what you can get. Sometimes they have books you would be willing to read for as low as $1.
In Manhattan (being a very literary town), there were always multiple street vendors hawking recent best sellers for $1. Again, book stores sold them off instead of returning them. Unfortunately, that means the author get zip for every one of those books sold! Still, it's hard to turn down a deal like that, especially given how much I used to read (I'd read walking to the subway, on the bus, at lunch, sitting on park benches, you name it).
For an author, who has no vote on the matter, it's like library books. Readers get them for free (or low cost), but the author gets his payment via word of mouth and personal recommendations (hopefully). Of course, for the vendors and booksellers who originally supplied them, it's as illegal as hell! (You'd really think the Devil would get all his paperwork in order before opening shop.)
Getting back to print formats, Print-on-Demand (PoD) books are generally higher quality and produced in lower volumes, while mass-market paperbacks are printed in volume, and get a substantially discounted rate. But I was just curious how much more expensive printing in 5x7 would cost.