Economics
The true cost of anything is whatever you gave up to get it
Every choice forecloses every alternative you could have made instead, and economists treat the value of the best forgone alternative as the real cost of a decision — not just the cash spent. Spending an afternoon watching a film 'costs' the film's ticket price plus whatever else you could have done with that afternoon. The concept, formalised by economists in the Austrian tradition in the late 19th century, is one of the most quietly load-bearing ideas in all of economics.