Economic value is a measure of the benefit provided by a good or service to an economic agent. It is generally measured relative to units of currency, and the interpretation is therefore "what is the maximum amount of money a specific actor is willing and able to pay for the good or service"?
Note that economic value is not the same as market price, nor is economic value the same thing as market value. If a consumer is willing to buy a good, it implies that the customer places a higher value on the good than the market price. The difference between the value to the consumer and the market price is called "consumer surplus". It is easy to see situations where the actual value is considerably larger than the market price: purchase of drinking water is one example.
The economic value of a good or service has puzzled economists since the beginning of the discipline. First, economists tried to estimate the value of a good to an individual alone, and extend that definition to goods which can be exchanged. From this analysis came the concepts value in use and value in exchange.
Value is linked to price through the mechanism of exchange. When an economist observes an exchange, two important value functions are revealed: those of the buyer and seller. Just as the buyer reveals what he is willing to pay for a certain amount of a good, so too does the seller reveal what it costs him to give up the good.
Additional information about market value is obtained by the rate at which transactions occur, telling observers the extent to which the purchase of the good has value over time.
Said another way, value is how much a desired object or condition is worth relative to other objects or conditions. Economic values are expressed as "how much" of one desirable condition or commodity will, or would be given up in exchange for some other desired condition or commodity. Among the competing schools of economic theory there are differing metrics for value assessment and the metrics are the subject of a "Theory of Value." Value theories are a large part of the differences and disagreements between the various schools of economic theory.
In neoclassical economics, the value of an object or service is often seen as nothing but the price it would bring in an open and competitive market. This is determined primarily by the demand for the object relative to supply in a perfectly competitive market. Many neoclassical economic theories equate the value of a commodity with its price, whether the market is competitive or not. As such, everything is seen as a commodity and if there is no market to set a price then there is no economic value.
In classical economics, the value of an object or condition is the amount of discomfort/labor saved through the consumption or use of an object or condition (Labor Theory of Value). Though exchange value is recognized, economic value is not, in theory, dependent on the existence of a market and price and value are not seen as equal. This is complicated, however, by the efforts of classical economists to connect price and labor value. Karl Marx, for one, saw exchange value as the "form of appearance" [Erscheinungsform] of value, which implies that, although value is separate from exchange value, it is meaningless without the act of exchange, i.e., without a market.
In this tradition, Steve Keen makes the claim that "value" refers to "the innate worth of a commodity, which determines the normal ('equilibrium') ratio at which two commodities exchange." To Keen and the tradition of David Ricardo, this corresponds to the classical concept of long-run cost-determined prices, what Adam Smith called "natural prices" and Karl Marx called "prices of production." It is part of a cost-of-production theory of value and price. Ricardo, but not Keen, used a "labor theory of price" in which a commodity's "innate worth" was the amount of labor needed to produce it.
"The value of a thing in any given time and place", according to Henry George, "is the largest amount of exertion that anyone will render in exchange for it. But as men always seek to gratify their desires with the least exertion this is the lowest amount for which a similar thing can otherwise be obtained." 
In another classical tradition, Marx distinguished between the "value in use" (use-value, what a commodity provides to its buyer), labor cost which he calls "value" (the socially-necessary labour time it embodies), and "exchange value" (how much labor-time the sale of the commodity can claim, Smith's "labor commanded" value). By most interpretations of his labor theory of value, Marx, like Ricardo, developed a "labor theory of price" where the point of analyzing value was to allow the calculation of relative prices. Others see values as part of his sociopolitical interpretation and critique of capitalism and other societies, and deny that it was intended to serve as a category of economics. According to a third interpretation, Marx aimed for a theory of the dynamics of price formation, but did not complete it.
In 1860, John Ruskin published a critique of the economic concept of value from a moral point of view. He entitled the volume Unto This Last, and his central point was this: "It is impossible to conclude, of any given mass of acquired wealth, merely by the fact of its existence, whether it signifies good or evil to the nation in the midst of which it exists. Its real value depends on the moral sign attached to it, just as strictly as that of a mathematical quantity depends on the algebraic sign attached to it. Any given accumulation of commercial wealth may be indicative, on the one hand, of faithful industries, progressive energies, and productive ingenuities: or, on the other, it may be indicative of mortal luxury, merciless tyranny, ruinous chicanery." Gandhi was greatly inspired by Ruskin's book and published a paraphrase of it in 1908.
Economists such as Ludwig von Mises asserted that "value," meaning exchange value, was always the result of subjective value judgements. There was no price of objects or things that could be determined without taking these judgements into account, as manifested by markets. Thus, it was false to say that the economic value of a good was equal to what it cost to produce or to its current replacement cost.
Value in the most basic sense can be referred to as "Real Value" or "Actual Value." This is the measure of worth that is based purely on the utility derived from the consumption of a product or service. Utility derived value allows products or services to be measured on outcome instead of demand or supply theories that have the inherent ability to be manipulated. Illustration: The real value of a book sold to a student who pays $50.00 at the cash register for the text and who earns no additional income from reading the book is essentially zero. However; the real value of the same text purchased in a thrift shop at a price of $0.25 and provides the reader with an insight that allows him or her to earn $100,000.00 in additional income is $100,000.00 or the extended lifetime value earned by the consumer. This is value calculated by actual measurements of ROI instead of production input and or demand vs. supply. No single unit has a fixed value.
The theory of value is closely related to that of allocative efficiency, the quality by which firms produce those goods and services most valued by society. The market value of a machine part, for example, will depend upon a variety of objective facts involving its efficiency versus the efficiency of other types of part or other types of machine to make the kind of products that consumers will value in turn. In such a case, market value has both objective and subjective components.