NEW YORK (Reuters) - As college attendance wanes through the semester, one professor teaches her 300 geoscience students how to work out the cost of cutting class.

A student walks on the campus of Yale University in New Haven, Connecticut November 12, 2015. REUTERS/Shannon Stapleton

Skippers waste $50 to $70 per lecture, depending on whether the student is in-state, said Julie Brigham-Grette of the University of Massachusetts at Amherst.

“Students need to understand how much money they are wasting when they don’t show up,” Brigham-Grette said. “If it’s $50 a class, then they should think, I just flushed that money down the toilet.”

The amount of money lost to class-skipping across the country per day is staggering, based on attendance figures. For Brigham-Grette, a good day is when 80 percent of her 300 students show up.

One 2014 study at Harvard University, using hidden cameras and self-reporting, found only 60 percent attendance on average at any given lecture. The roll call declined as the semester went on, from 79 percent at the beginning to 43 percent at the end.

According to the site StudentScholarshipSearch (bit.ly/20GO183), the overall cost of skipped classes during a college career is $6,586 per student in a private institution, and $2,400 for public colleges.

Doing the math is not all that difficult. And it can work for spending on classes for younger children, or summer camp, which can cost $100 per day. That is valuable information for parents to share when kids express the urge to stay home and watch cartoons.

Simply divide the tuition amount by the total class hours, and work in things like extracurriculars to get a number that makes an impression on the student, said Claudia Sangster, director of family education and governance at Northern Trust Corp.

Stephanie Kibler worked out her per-class costs in a spreadsheet in 2007 when she was studying at the Rochester Institute of Technology, and then shared it on her blog (bit.ly/1P0v7Z1).

The total was $52.51 per hour, based on her tuition and amount of homework.

Kibler then figured out her loan costs which led her to take a leave of absence to work and build up more savings.

Now a museum technician and copywriter in Fairfax, Virginia, Kibler, 29, said that knowing what college cost her per hour was magical.

“Once I had that number, it was a little harder to skip classes, unless I was doing something extremely worthwhile with the time. After all, if it’s a two-hour class, did I really want to blow over $100 by skipping it?” Kibler said.

And after working several years, the value of $52.51 per hour became clearer to her: “If that were an hourly rate for a job, that would be a $105,000 per year.”

She was making $7.25 per hour at her work-study job in college.

The knowledge has also helped her overall approach to money as an adult.

“Everything I learned about money stemmed from me digging into my finances in college, figuring out where I stood and just what was going on inside my personal financial machine,” Kibler said. “I learned how to be careful with money, to plan and save, but most importantly, how to make my money work for me so that I can use it to live the life I want, even despite those student loans.”

As for Brigham-Grette, she said her students appreciate the math exercise, but the scientist had yet to calculate if that translated into higher attendance.

Her students should be on the lookout, though: She is thinking of adding a question about the cost of skipping class to her final exam.