It took me awhile to think of a way to respond to this, then I remembered I.I. Rubin's book Essays on Marx's Theory of Value. Chapter 5 seems to answer your question pretty well.
"At first glance all the basic concepts of Political Economy (value, money, capital, profit, rent, wages, etc.) have a material character. Marx showed that under each of them is hidden a definite social production relation which in the commodity economy is realized only through things and gives things a determined, objectively-social character, a "determination of form" (more precisely: a social form), as Marx often put it. Analyzing any economic category, we must first of all point to the social production relation expressed by it. Only if the material category is an expression of a precisely given, determined production relation, does it enter the framework of our analysis. If this material category is not related to a given production relation among people, we pull it out of the framework of our analysis and set it aside. We classify economic phenomena into groups and build concepts on the basis of the identity of the production relations which the phenomena express, and not on the basis of the coincidence of their material expressions. For example, the theory of value deals with exchange between autonomous commodity producers, with their interaction in the labor process through the products of their labor. The fluctuation of the value of products on the market interests economists not for itself, but as it is related to the distribution of labor in society, to the production relations among independent commodity producers. For example, if land (which is not the product of exchange) appears in exchange, then production relations in this case do not connect commodity producers with commodity producers, but with a landowner; if the price fluctuations of plots of land have a different influence on the course and distribution of the production process from the price fluctuations of the products of labor, then we are dealing with a different social relation, a different production relation, behind the same material form of exchange and value. This social relation is subject to special analysis, namely in the context of the theory of rent. Thus land, which has price, i.e., a money expression of value (as a material category), does not have "value" in the sense mentioned above, i.e., in the act of exchange the price of land does not express the functional social relation which relates the value of the products of labor with the working activity of independent commodity producers. This led Marx to the following formulation, which has often been misinterpreted: "Objects that in themselves are not commodities, such as conscience, honor, &c., are capable of being offered for sale by their holders, and of thus acquiring, through their price, the form of commodities. Hence an object may have a price without having value. The price in that case is imaginary, like certain quantities in mathematics. On the other hand, the imaginary price-form may sometimes conceal either a direct or an indirect real value relation; for instance, the price of uncultivated land, which is without value, because no human labor has been incorporated in it" (C., I, p. 102). These words of Marx, which have often puzzled and even provoked the mockery of critics, express a profound idea about the possible divergence between the social form of working relations and the material form which corresponds to it. The material form has its own logic and can include other phenomena in addition to the production relations which it expresses in a given economic formation. For example, in addition to the exchange of products of labor among independent commodity producers (the basic fact of the commodity economy), the material form of exchange includes the exchange of plots of land, the exchange of goods which cannot be multiplied by labor, exchange in a socialist society, etc. From the standpoint of the material forms of economic phenomena, the sale of cotton and the sale of a painting by Raphael or a plot of land do not in any way differ from each other. But from the standpoint of their social nature, their connection with production relations, and their impact on the working activity of society, the two phenomena are of a different order and have to be analyzed separately."
marxists.org/archive/rubin/value/index.htm