Whenever you make a choice among 3 choices, and sometime after the decision evaluate each choice with the benefit of hind sight, you get a concrete idea of the advantage, gain, profit, ROI, benefit of each choice.

Suppose you manage to attach a concrete value to each choice. The values of the other 2 choice you didn’t take are loosely known as opportunity costs.

If you quit your job and stay home, the previous salary (eg 7k) is an opportunity cost