Opportunity cost is the invisible price tag hanging from every yes.

Every agreement has a price that is never quoted, made up of the things you will now decline, delay, or do badly. Because it is never stated, saying yes feels free in the moment and expensive three weeks later. Making the cost explicit at the point of decision is the whole discipline.

When you are about to agree to something, ask what it costs in hours and what those hours were otherwise for. Do it before answering. The delay alone prevents a lot of regret.