Marx has explained that the basis of the value of commodities is the amount of labour they contain. He goes on to look at how this value is a social product…
The total labour power of society, which is embodied in the sum total of the values of all commodities produced by that society, counts here as one homogeneous mass of human labour power, composed though it be of innumerable individual units. Each of these units is the same as any other, so far as it has the character of the average labour power of society, and takes effect as such; that is, so far as it requires for producing a commodity, no more time than is needed on an average, no more than is socially necessary.
The way a commodity appears on the market, as a seemingly independent useful object with a price tag, may make it appear that value is some sort of innate property of the commodity. But Marx indicates that the commodity’s value is based on its share of the overall productiveness of the society that produces it.
The labour time socially necessary is that required to produce an article under the normal conditions of production, and with the average degree of skill and intensity prevalent at the time. The introduction of power-looms into England probably reduced by one-half the labour required to weave a given quantity of yarn into cloth. The hand-loom weavers, as a matter of fact, continued to require the same time as before; but for all that, the product of one hour of their labour represented after the change only half an hour’s social labour, and consequently fell to one-half its former value.
The ‘socially necessary labour time’ to produce a given commodity will differ across different societies due to their diverse conditions of technological development, and other production conditions such as wages and expected quality of life for workers etc. The socially necessary labour time required for the production of a given commodity in a specific society is what determines the social value of that commodity…
We see then that that which determines the magnitude of the value of any article is the amount of labour socially necessary, or the labour time socially necessary for its production. Each individual commodity, in this connexion, is to be considered as an average sample of its class.
So despite the market appearance of the commodity as a seemingly free market entity with its own price tag, its value is actually an expression of its share of the overall value created by the labour in its society.