The cost of the next best opportunity foregone.
If you find it hard to determine the cost of something, you need to look at what opportunity you miss out on by doing or acquiring that something. For example if you swap your hat for a cat, how much is the cat worth? Well, you lost the opportunity to sell your hat for money. If you had sold you hat for money, you would've received
x. So the opportunity cost of the cat is
x. The opportunity cost is a useful way to put a lower bound on the cost of something that is otherwise hard to put a price on.
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