Compute opportunity cost as the value of the next-best alternative forgone.
The true cost of anything is not its price tag — it's what you gave up to get it. Economists call the value of the NEXT-BEST alternative you didn't choose the opportunity cost. Take the Saturday job shift and the cost isn't "zero because I earned money" — it's the game you missed. Choose college and the cost includes four years of salary not earned, on top of tuition.
The discipline is naming the single next-best alternative — not everything you didn't do, just the best one. This lens changes decisions: "free" events cost your afternoon; a "cheap" purchase costs whatever else that money was second-best for. People who think in opportunity cost make quietly better choices their whole lives.
Mastery looks like: They instinctively answer "what would you give up?" before "what does it cost?" on real decisions.
Common stumbles: Summing every forgone alternative; ignoring time costs because no money moved.