A cadre of so-called open-education publishers is slowly gaining the trust of schools and university systems by asking them this question: Why pay $80 for a textbook that might quickly become outdated, when you could pay $5 or less?

Using free, open-source education materials, firms such as CK-12 Foundation and Boundless are building digital textbooks and learning materials (mostly for math and science) that students and teachers can use and edit as they wish, Fortune reported.

The U.S. spends more than $7 billion every year on K-12 textbooks, and college textbook prices have increased by a whopping 812 percent since 1978, Fortune reported, surpassing inflation, college tuition increases and even the much-discussed rise in medical expenses during that time period.

College students report that they pay an average of $655 a year on books and supplies, according to a 2012 report from the National Association of College Stores. Read more.