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RALEIGH — It costs a lot more to train a future engineer than to train a future journalist. Some smart aleck might suggest the cost differential is entirely understandable, since a poorly trained engineer will tend to wash out of her profession while a poorly trained journalist might well rise to the top of his.

Since my undergraduate degree was in journalism, I will not be that smart aleck.

What I will suggest, however, is that policymakers pay greater attention to the full cost — not just the net price — of obtaining college degrees. A new study coauthored by UNC-Chapel Hill professor Steven Hemelt offers some useful insights about the issue.

Hemelt and his colleagues pulled 17 years of data from a national survey of instructional costs across public and private institutions. Focusing on 20 major fields of study that collectively account for most student enrollments, they found large differences in instructional costs. Courses in electrical engineering, for example, cost an average of $434 per credit hour. Courses in communications and media averaged $185 per credit hour.

While other technical disciplines such as nursing ($375 an hour), mechanical engineering ($372), physics ($281) and computer science ($274) also had higher-than-average costs, the researchers found that the need to outfit classrooms with high-tech equipment does not explain much of the variance in instructional costs. The key factors are how much professors and instructors are paid and how many students are in their classes.

Yes, it’s often more expensive for universities to recruit teachers in technical fields for which there are lucrative alternatives such as private industry or research labs. Individuals with math, science or economics degrees can often make more outside of the academy than inside it, while the same cannot necessarily be said for those with degrees in education, history or English.

But that’s not the only consideration. While professors in economics, political science and business departments receive relatively high salaries, their instructional costs per credit hour come in closer to the middle of the pack because their classes are relatively large.

Education professors and instructors, for example, make an average of $80,340 a year while their counterparts in economics departments make $123,720. In terms of instructional cost per credit hour, however, education degrees are more expensive ($291) than economics degrees ($218). That’s because economics classes tend to be significantly bigger than education classes. In engineering and nursing programs, however, classes must necessarily be smaller, so their higher professor salaries translate into higher costs per student.

Hemelt and his coauthors also looked at trends over time. While real instructional costs as a whole didn’t rise much from 2000 to 2017, there was, again, considerable variation. In some high-demand STEM fields, costs per credit hour actually went down quite a bit as enrollments surged and universities chose to increase class sizes or hire part-time adjuncts rather than bring on a proportionate number of costlier tenure-track professors. On the other hand, in such fields as history, sociology, education and the arts, instructional costs per student credit hour have gone up.

How about online instruction? So far, the researchers discovered, it hasn’t really moved the needle. “We find some evidence that an increase in the share of undergraduate coursework completed online is related to lower salary costs,” they wrote. “But estimates for the other cost drivers suggest that any short-run cost savings on salaries are offset by smaller class sizes and an uptick in non-personnel expenditures.”

The cost of a given program is, of course, only one side of the ledger. What are the benefits? I think we can all agree that producing well-educated, highly trained and innovative engineers is worth a significant investment of time and money. For other disciplines, though, high and rising instructional costs are harder to justify.

Inevitably, universities will respond by increasing class sizes and making greater use of adjuncts. And non-academic institutions will respond by offering alternative means of teaching and certifying job skills. Both responses make sense to me.

John Hood is a John Locke Foundation board member and author.