An overwhelming majority of millennials surveyed regret their student debt, and it's no surprise why so many of them have the same answer

• Only 15% of millennials with student loan debt don't regret their student debt, according to a survey by Business Insider Intelligence.
• Many millennials overestimated the salary of their first post-grad job, thinking they'll be able to afford monthly payments.
• The cost of college has more than doubled in the past decade - which has made a degree less advantageous than it once was, one expert told Business Insider.
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Millennials are a regretful bunch, at least when it comes to their student loan debt.

Only 15.8% of student-loan-indebted millennials surveyed said they don't regret taking on their student debt, according to the "Master Your Money: Learn and Plan Survey", conducted by Business Insider Intelligence.

The online survey polled 2,007 American millennials born between 1982 and 2000, fielded to a third-party sample between November 22 and November 27, 2019. Out of the total surveyed, a total of 1,219 had taken out student loans.

The remaining respondents had some level of regret about their student debt: Another 15.8% felt slightly regretful, while 24% felt moderately so. That left nearly half swimming in deeper regret: 20.8% said they're very regretful, while 23.5% said they feel extremely regretful.

Those doubts are not surprising, considering that the cost of college has more than doubled since the 1980s. More students have had to borrow money to keep up with these increases, sending the total student debt owed by Americans past $1.5 trillion. The average student-loan debt per graduating student in 2018 who took out loans is $29,800.

Is the cost of college worth it?

It doesn't help that some millennials are taking out student loans under a misconceived perception of their future earnings.

According to a separate survey by personal finance company SoFi, nearly half of millennials said the biggest mistake they made with their student loans was overestimating the salary of their first job out of college and assuming they'd be able to afford their monthly payments.

The average millennial earns $35,455, according to a SuperMoney report that analyzed US Census Bureau data. That's only $5,000 more than the average student loan debt per graduating 2018 student.

Together, the Business Insider and SoFi surveys paint a bigger picture: The advantages of a college degree may not outweigh the ever-increasing costs required to get one - at least, not for everyone.

"The rewards for college have expanded and grown from 1985 to a little after 2000 and sort of leveled off in the past decade," Richard Vedder, an author and distinguished professor emeritus of economics at Ohio University, previously told Business Insider.

The "advantage of a degree today is less than it was 10 years ago, because of the rising cost," he added. "The return on investment has fallen."