Rising tuition costs aren’t the only thing driving up the cost of higher education. Textbook prices have skyrocketed in recent years.

Since 2006, the cost of a college textbook has increased by 73 percent — or more than four times the rate of inflation — according to Covering the Cost, a new report from the non-profit Student PIRGs (Public Interest Research Groups). It’s not uncommon for an individual book to cost more than $200, and some have price tags that go as high as $400, the report said.

Exact numbers are hard to come by, but the College Board recommends that students budget about $1,200 a year for textbooks and supplies.

“This is a serious problem,” said Ethan Senack, higher education advocate at Student PIRGs. “We’ve known for a long time that high textbook prices create a lose-lose choice for students. They can either spend hundreds of dollars to buy the textbook, take time away from studying to work extra hours to pay for their books, or they can go without the book and accept the consequences.”

Imagine attending classes without the required textbooks? Max Alphonse, a member of Student PIRGs and a senior at Fitchburg State University in Mass., has sometimes done just that.

“I’ve had to decide whether to get the book or just skip it because it cost too much,” he said. “It does eat into my financial aid.”

Alphonse told NBC News that when he takes courses without a required book, he tries to borrow it from another student.

Read More: College Textbook Prices Have Risen 1,041 Percent Since 1977

The PIRG report blamed soaring prices on two key factors:

Lack of competition: Five publishers control 80 percent of the market

No consumer choice in this purchase: Students must buy the books they’re assigned

The Association of American Publishers (AAP) questions the PIRG report and its findings. David Anderson, AAP’s executive director of higher education, said the study is “way off base” about the financial pressures that students and parents face. He cites figures from the National Association of College Stores that shows the typical student now spends around $560 per year on required course materials, down from $700 in the 2007/2008 academic year.

The industry is changing, Anderson said, moving away from printed textbooks to digital learning programs.

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“I would agree that the price of hard-bound print books is high, and this industry understands that and they are moving to make new digital resources available that are more engaging, help students learn and get better grades, and slash the prices in half or more,” Anderson told NBC News. “I think we’re the only industry involved with higher education that can say they're cutting costs by half by shifting in this digital direction.”

Anderson said that the nation’s publishers are working with college and university bookstores across the country to offer students discounts of up to 70 percent.

How are today’s book costs affecting students?

For its report, Student PIRGs surveyed 5,000 students on campuses across the country to find out how they’re dealing with the high cost of textbooks. The survey’s key findings:

Nearly one-third (30 percent) of the students responding use financial aid money to buy their required books.

Of those who did use financial aid dollars on books, the average amount spent was more than $300 per semester.

Community college students are nearly twice as likely to use financial aid for books as students at four-year private or public schools.

Senack told NBC News he finds these results troubling, since financial aid is typically applied to tuition or room and board before being used for other expenses. Based on this survey, he estimates that college students across the country are spending about $1.5 billion every semester in financial aid on books.

Read More: New Bill in Congress Would Help Make College Textbooks Free Online

“We think it’s absolutely egregious to be spending that much on textbooks and using that much financial aid when there are alternatives that could save students money and put it back into their higher education,” Senack said. “If you look at the average hours working to buy these textbooks, a student today would have to work 28 hours just to buy a single textbook.”

Kevin Corcoran, strategy director for the Lumina Foundation, calls the cost of college books “a significant problem” for many students today. His organization is working to increase the proportion of Americans who earn higher education degrees and certificates.

“Faculty members choose the books, and a lot of times they have no idea how much those books cost. If they did, I think they’d be pretty concerned,” Corcoran said. “If you’re a community college student in a lot of places right now, the cost of the textbooks can exceed the tuition you’re paying.”

The free alternative

There are ways to lower the cost of books. Students can buy used, rent from a variety of websites that offer savings of up to 90 percent, or buy new books and try to sell them back at the end of the semester.

A growing number of colleges and universities across the country are now using “open-source” textbooks. These books are written by faculty and peer-reviewed, just like traditional books, but they’re free online and free to download. They’re typically available in print for between $20 and $40.

In 2013, Tidewater Community College in Norfolk, Va., made a bold decision and offered an associate’s degree program that does not use traditional books. The typical student at Tidewater spends about $3,400 on books and supplies during his or her years at the school. According to the administration, that’s about one-quarter of the expense of attendance.

Students enrolled in the Z-Degree program (the "z" stands for “zero textbooks”) take Z-Courses that use all open-source materials.

“We saw textbook costs as a barrier to our students’ success,” said Linda Williams, a professor of business management and administration, who helped develop Tidewater’s Z-Degree. “My colleagues and I are faced every semester with students who find themselves unable to afford the course materials. Those students either do extremely poorly in the course because they’re trying to cobble together some form of content on their own or they drop the class or withdraw from school.”

Read More: How Rising Textbook Prices Mirror Rising Drug Costs

Williams tells NBC News the Z-Courses have been extremely well received by students, who often say they’re doing better without textbooks. The response from the faculty has been so positive that the school plans to offer three more Z-Degree programs this fall.

“We want to provide our students with options,” Williams said. “The same way students can take courses face-to-face, online or hybrid, we truly believe that we should be able to provide the option of taking the course as a Z-course without the purchase of traditional textbooks.”

More than 180 open-source textbooks are available right now. If every student were assigned just one in place of a traditional book, it would save students across the country more than $1billion on textbooks each year, based on the Student PIRG’s calculations.

“But that potential won’t be realized unless the higher education community takes an active role in transitioning away from traditional publishers,” Senack said. “We need a more broad commitment from the higher education community to increasing access to open educational resources.”

Herb Weisbaum is The ConsumerMan. Follow him on Facebook and Twitter or visit The ConsumerMan website.