A dynamic computational model of a simple commodity economy is examined and a theory of the relationship between commodity values, market prices and the efficient division of social labour is developed. The main conclusions are: (i) the labour value of a commodity is an attractor for its market price; (ii) market prices are error signals that function to allocate the available social labour between sectors of production; and (iii) the tendency of prices to approach labour values is the monetary expression of the tendency of a simple commodity economy to allocate social labour efficiently. The model demonstrates that, in the special case of simple commodity production, Marx's law of value can naturally emerge from multiple local exchanges and operate ‘behind the backs’ of actors solely via money flows that place budget constraints on their local evaluations of commodity prices, which are otherwise subjective and unconstrained.