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.
Only give up a thing when you want some other condition so much that the thing no longer has any attraction for you, or when it seems to interfere with that which is more greatly desired.
Mohandas Gandhi, quoted in Gandhi: His Life and Message for the World by Louis Fischer.
Because people face trade-offs, making decisions requires comparing the costs and benefits of alternative courses of action. In many cases, however, the cost of an action is not as obvious as it might first appear. […] The opportunity cost of an item is what you give up to get that item. When making any decision, decision makers should be aware of the opportunity costs that accompany each possible action. In fact, they usually are. College athletes who can earn millions if they drop out of school and play professional sports are well aware that their opportunity cost of college is very high. It is not surprising that they often decide that the benefit of a college education is not worth the cost.
N. Gregory Mankiw, Principle of Economics (6th ed., 2012), Ch. 1. Ten Principles of Economics.
Popular saying dating back to the 1930s; sometimes attributed to Milton Friedman.
If the economy is producing efficiently, scarcity values must be equal to opportunity costs, and their common value is the efficiency price... An economy is efficient, as opposed to just production efficient, if it is impossible to make anyone better off without making someone else worse off. In addition to producing efficiently, the final consumers must have exhausted all possibilities of mutually beneficial exchange. This in turn requires they all face the same market prices and that these are equal to efficiency prices...The case for removing distortions and moving market prices closer to efficiency prices rests on the argument that prices influence production efficiency and the reform will increase production efficiency.
World Bank (1983), World Development Report, p. 42