The object of the law of value is to elucidate the actual exchange relations of commodities.
Value manifests itself as exchange value, as a quantitatively determined relationship, in virtue of the fact that one commodity can be exchanged for another.
Value is consequently the necessary theoretical starting point whence we can elucidate the peculiar phenomenon of prices resulting from capitalist competition.
The expansion of the market creates a need for enhanced and more regular supply, and this in turn impels commercial capital to acquire control of production as well.
It is obvious, moreover, that the formation of price in capitalist society must differ from the formation of price in social conditions based upon the simple production of commodities.
It is therefore utterly false to say that Marx revokes the law of value as far as individual commodities are concerned, and maintains it in force solely for the aggregate of these commodities.
Since, however, the reduced surplus value is to be distributed among them in like manner, the modification of their respective parts in the production of surplus value must find expression in a modification of the prices.
As soon, however, as capitalist competition has definitively established the equal rate of profit, that rate becomes the starting point for the calculations of the capitalists in the investment of capital in newly-created branches of production.
But whether, for example, a coat can be exchanged for twenty yards of linen cloth or for forty yards is not a matter of chance, but depends upon objective conditions, upon the amount of socially necessary labor time contained in the coat and in the linen respectively.