In the economic sense, not in the moral sense, value is synonymous with utility, which is defined in relative terms using choice. It is thought that "value" is related to the satisfaction that an item brings to a consumer, as in, if the consumer's wants or needs are "not satisfied" by "not having" a product, then the customer values the product.
See also Utility.
Note that the price you pay is not the "value" of the thing. Compare the buyer and the seller: the buyer places a higher value on the thing than the price paid (the difference here is called the 'consumer surplus'), and the seller places a lower value on the thing than the price paid (this is called 'markup'). If it weren't for these two facts being true, then the exchange would never take place. So, while value can be determined via utility, it cannot be inferred from price.
An economist is someone who knows the price of everything and the value of nothing.
(Oscar Wilde said this, but about cynics, not economists.)
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