The True Cost of Doing Something Is the Next Best Alternative

The true cost of doing something is the next best alternative. What would you do in place of the thing you're doing? That's what you're going to give up. There's an opportunity cost to eating a doughnut, if i'm trying to limit my calories. The opportunity cost of the doughnut is the beer. People won't waste your time if you know you're keeping track of and thinking about what you could be doing. Otherwise, it's quite useful to know what you're giving up.