Most graduate students get at least some amount of grand money from state and or federal as well as from the university itself. Mostly they tend to serve as TA's if not teaching the classes themselves. So make contribution to a open source books mandatory. Make them write and revise the very textbooks their university uses. They do at least partially teach/tutor other students so they are relatively well informed as to how to improve the material as far as how the class is taught.
Also by using open source books the classes could opt to use more textbooks in their classes. For instance one textbook might explain something better to one student than another.
Also wouldn't the money the state spends on k-12 textbooks be better spent on grad students to write the books?