The College Board, a not-for-profit association, calculated in a 2010 report (based on 2008 data) that a typical student who enters a four-year college at age 18 and borrows his way through earns enough by age 33 to make up for his costs, including foregone wages and loan interest.

If a bond paid for itself that quickly, the return would be between 5% and 6% a year. That’s a handsome payoff; stocks have historically returned around 7% a year after inflation. And it says nothing of college’s other benefits, such as enlightenment, fun and higher job satisfaction.

Two big caveats: The College Board math assumes everyone goes to a public college. Those usually cost less than private ones—often a lot less—and that skews returns higher. The report also doesn’t account for dropouts or extra college years. Only 56% of students who enroll in a four-year college earn a bachelor’s degree within six years, according to a report last year by the Harvard Graduate School of Education.

PayScale, a Seattle data firm, examines the links between pay and variables like colleges and majors. Its analysis, which also ignores dropouts but accounts for students who take longer to complete their degrees, finds an average yearly return of 4.4% for degrees from 853 schools. That assumes students get financial aid, as most do.