> Under simplifying assumptions, including competitive labor markets, no direct schooling costs, and constant proportional returns, this specification yields an interpretable estimate of the average percentage increase in earnings associated with an additional year of schooling.

So, an individual student will be expected to have higher earnings with more schooling. But as a matter of public policy that involves taxpayer funding of higher education, I cannot accurately base those decisions on a methodology that assumes a cost of zero ("no direct schooling costs").

Your source also identifies that higher marginal returns are found in lower-income, lower-educated countries, which makes sense. How much marginal return (for the taxpayer dollar) can we expect in a high-income country with a service-based economy and a third of the population already having a bachelor's degree?