I offer a text, Linear Algebra, both for Free download and for sale (at Amazon $22). It generates a respectable number of sales. People will pay, or at least some people will pay, for quality. They do get mad, though, at paying hundreds.

I am not a business person but I can imagine a scenario where schools purchase a block of the materials for classes. Say, a prof wants to use a Graph Theory document. Their bookstore pays $13 each for the materials, and a bill for that appears on the student's college charge. Seems possible to me. And beyond that individuals could buy one at a time, of course.