“How much do you think it costs to go to college? Most people are likely to answer by adding together their expenditures on tuition, room and board, books, and the like, and then deducting any scholarship funds they may receive. Suppose that amount comes to $15,000.
Economists keep score differently. They first want to know how much you would be earning if you were not attending college. Suppose that salary is $20,000 per year. This may seem irrelevant, but because you give up these earnings by attending college, they must be added to your tuition bill. You have that much less income because of your education. On the other side of the ledger, economists would not count all of the university’s bill for room and board as part of the costs of your education. They would want to know how much more it costs you to live at school rather than at home. Economists would count only these extra costs as an educational expense because you would have incurred these costs whether or not you attend college. On balance, college is probably costing you much more than you think. And, as we will see later, taking opportunity cost into account in any personal planning will help you to make more rational decisions.”

William Baumol and Alan Blinder, Economics: Principles and Policy (2011), Ch. 1 : What is Economics?