Money & WorkMental model
The real price of anything is what you give up for it.
Opportunity cost is the value of the best alternative you don't choose. A €20,000 car also costs what that money would have earned elsewhere: at a 5% return it would be roughly €32,600 after 10 years, a gain of €12,600. Time has opportunity costs too: two hours of price comparison to save €5 is an hourly wage of €2.50.
Useful: For every expense, ask not just what it costs but what you'd have gotten instead.
Source: de.wikipedia.org