[ken]

"This is an engineering textbook, about 300 pages long, OK? Hard-bound. This costs, anybody guess? How much would it cost in a bookstore? OK, this cost $22 to the students. Why does it cost $22? Because it's published on-demand, and it's developed from this repository of open material. If this book was to be published by a regular publisher, it would cost at least $122 dollars."

-- Richard Baraniuk, 2006 -- http://www.ted.com/index.php/talks/richard_baraniuk_on_open_...

So it's a bit more complex than that. It may be "economy of scale" in that a professor isn't going to take the time to write a solid textbook without being able to make back at least his expenses. But if we had some way for people to collaborate more easily and cheaply to write a textbook...