But skeptical student advocates say the benefits of some programs can be murky.

Discounts can be hard to quantify because the original price of the materials is unclear, according to a report released on Thursday by the U.S. PIRG Education Fund, an advocacy group, which examined dozens of contracts between publishers and more than 30 two- and four-year public colleges. Also, some contracts allow publishers to reduce the discounts if too few students participate. In some cases, quotas to maintain lower pricing are as high as 90 percent of students taking a course.

“The idea of discounts sounds good on its face, but the contracts aren’t living up to the hype,” said Kaitlyn Vitez, higher education campaign director for the education fund and the author of the report.

Further, the report found, it’s not always obvious how students can opt out of the billing if they want to search elsewhere for textbooks.

“Students should always have a choice of how to pay,” said Nick Sengstaken, a senior at the University of North Carolina at Chapel Hill who is a member of the student government and an advocate for textbook affordability.

Pearson, the dominant education publisher in the United States, has inclusive access agreements with about 900 institutions, said Nik Osborne, the company’s senior vice president of strategy and business operations. Not all courses offer the option. Typically, a professor or department elects to participate for a specific class, he said. Students must be allowed to opt out, he said, but relatively few do because the cost is low.