Every choice has a hidden price tag: whatever you didn't pick instead

Opportunity cost is the value of the next-best option you give up when you make a choice — not just the cash you spend. Spending an evening studying costs you that evening's sleep or socializing; a factory devoting a production line to one product costs it the revenue the other product would have earned. Austrian economist Friedrich von Wieser formalized the concept in the late 19th century, and it now underlies nearly every basic economic trade-off analysis.

— Friedrich von Wieser, Der natürliche Werth (Natural Value) — 1889