While Yale and its peer universities have been announcing their sticker prices for the upcoming academic year, these figures do not always tell the whole story.

Tuition, room and board at Princeton for this academic year undercut Yale’s term bill by $480, and next year that margin will grow to $1,220, making it the cheapest Ivy League school for the second straight year. But that gap may not be as large as it seems: Mandatory fees, though they are not included in Princeton’s sticker price, add hundreds of dollars to the cost of a year there.

At Princeton, these expenses help fund residential college events, class dues and student government activities. Indeed, every Ivy League school levies some kind of additional fee to help fund undergraduate life, and most include those fees that are mandatory in their total sticker price. But Yale and Harvard are the only Ivies that allow students to opt out of these added costs.

Yale’s optional student activities fee — which covers club sports teams, Yale College Council events and programs and student organizations through the Undergraduate Organizations Funding Committee — rose to $75 this year. Princeton’s fees — which include class dues, an activities fee that supports the student government and a fee for those students who live in the university’s residential colleges — totaled $835 for freshmen this year, said Robin Moscato, director of financial aid at Princeton. These expenses remain the same or increase modestly each year, she added.

“These fees make many of the services students use on a regular basis possible and are vitally important to our campus atmosphere,” Princeton student government president Michael Yaroshefsky said in an e-mail, adding that the revenue supports parties every year, among other events and programs.

Many of Princeton’s fees do not apply across all four years, Princeton spokeswoman Cass Cliatt said, which would make them difficult to bundle with the rest of the term bill.

“Tuition is the only mandatory fee from the University across all four classes,” Cliatt said in an e-mail.

Princeton’s residential college fees, for example, support many of the same activities Yale’s residential colleges offer: study breaks, social events and other programming, but only for the students — mostly freshmen and sophomores — who participate in Princeton’s residential college system.

Yale’s residential colleges receive funding for such events from the Provost’s Office and their own endowments, Provost Peter Salovey said. Princeton’s must rely on the fees.

Though Princeton released next year’s tuition, room and board expenses last month, the extra fees are usually set in the late spring or over the summer, Cliatt said.

But the other seven Ivy League universities either fold their mandatory student fees into the sticker price or allow students to opt out of paying the fees altogether.

Students at both Harvard and Yale pay an optional student activity fee (Harvard calls it an undergraduate council fee) of $75 each year. While Yalies can choose not to pay the extra cost by adjusting student account settings online, Cantabs must write a letter to Harvard administrators to opt out.

“We definitely try to encourage as many students as we can to pay it, because the more students that participate, the more money there is for student activities,” YCC treasurer Adam Thomas ’12 said, “but students have a choice, so that if they really don’t want to pay, they don’t have to.”

Thomas said the YCC uses the revenue from the fee, along with funding from the Yale College Dean’s Office and the President’s Office, to pay for events including the Winter Show and Spring Fling and programs such as the new bike share initiative. When funding from both offices was reduced to save University funds last year, the fee was increased to $75, though it had held steady at $50 since its establishment in 2005, he said. Income from the fees supplies more than two-thirds of the YCC’s budget.

Yet a quarter of the Yale student body chose not to pay the fee last year, former YCC president Rich Tao ’10 told the News in April, creating what he called a “free-rider problem”: Students who do not pay the fee can still go to YCC events.

Eight out of 31 students interviewed Wednesday said they chose to waive the fee. Two of the students who voluntarily chose to pay the fee said they felt they should contribute to the activities they take part in.

“My mom said I should support the activities of my classmates,” Lisa Wang ’12 said. “It’s part of a Yale education, I guess.”

Two students, both on financial aid, said they waived the fees because it saved them money.