Marx: "Let us now take two commodities, for example corn and iron. Whatever their echange relation may be, it can always be represented by an equation in which a given quantity of corn is equated to some quantity of iron, for instance 1 quarter of corn == x cwt of iron. What does this equation signifify? It signifies that a common element of identical magnitide exists in two different things, in 1 quarter of corn and similarly in x cwt of iron. Both are therefore equal to a third thing, which in itself is niether the one nor the other. Each of them, so far as it is exchange-value, must therefore be reducible to this third thing. [...] As use-values, commodities differ above all in quality, while as exchange-values they can only differ in quantity, and therefore do not contain an atom of use-value. If then we disregard the use-value of commodities, only one property remains, that of being products of labour."
In other words, Marx excludes use-value (what normal economists call "utility") as being a factor in the price of a commodity, apart from needing to exist at all (he reduces its relationship to a boolean relationship,) and makes it a function of labor quantity. This is so central to his argument that it comes up in the first few pages of Capital.