There's an old joke that says textbook prices are so out of line that economics students who buy the book should automatically fail the class.

In a challenge to the rising cost of college textbooks, more than 700 math and physics professors from 150 universities have issued a protest letter to a major publishing firm. The letter asks Thomson Learning to stop releasing frequently revised editions of books, a practice that hikes prices and phases out cheaper used versions.

The letter was organized by the California Public Interest Research Group and drafted by Dr. Michael Dennin, chair of the University of California at Irvine's Physics Department Undergraduate Committee.

"The canonical way of teaching mechanics doesn't change much, so we don't need new editions of the book every three years, which is what the turnover rate currently is," said Rainer Wallny, an assistant professor who teaches an introductory physics course at UCLA. Wallny was one of 20 professors at UCLA who signed the letter.

The average college student currently spends $900 a year on textbooks, which amounts to almost 20 percent of their total college tuition, CALPIRG claims. On average, new editions cost almost 60 percent more than their used counterparts, and most professors surveyed by CALPIRG said the new editions were justified less than half the time.

Many new textbooks also come "bundled," a practice that drives up the cost of the materials by packaging the editions with additional (and often unnecessary) supplements like CD-ROMS, study guides and online tools.

CALPIRG began researching the issue two years ago, according to Merriah Fairchild, co-author of Ripoff 101, the organization's report on textbook prices. The report, first released in January 2004 and then revised this past February, delved into how the current practices of the textbook industry are unnecessarily driving up the cost of college textbooks.

"The publisher's own industry numbers show the costs of textbooks are rising at four times the rate of inflation," Fairchild said.

The quick turnover rate also hits students' pockets by stymieing campus bookstores from buying back the used texts at the end of the semester. Fifty-nine percent of students report they were unable to find even one used book for their classes.

Fairchild said CALPIRG will embark on a report that will compile a list of alternatives that faculty can use in lieu of the typical textbook. Many professors have deviated from the norm by writing their own texts and posting them online for students to access for free.

"It is possible to cut the publishers out of the process altogether," she said. "And if they are really serious about listening to faculty the way they say they do, then they'll respond to the letter by ending this unnecessary practice."