Because an exchange, in its essence, is a relationship of equality where the subjectivities of the contracting parties neutralize each other. The objective price (as opposed to "subjective") is established by the very mechanism of the market: no enterprise can sustainably sell below its cost price, and competition prevents it from selling above it.
Prices are merely as exchange values quantified in monetary terms, which make supply and demand only a secondary controller of prices. In terms of micro-economy, the emphasis made on the law of supply-and-demand and the intertwined marginal cost theory have disconnected economic science from managerial and accounting practices. In the later field, fully allocated costs and margins are the main drivers. The Objective Political Economy provides a theory of price that connects back accounting practices to forward looking of managerial decisions based on true ROI, based on actual solution on cost of production, and the allocation of common costs bounded to enterprise investments (see equi-management).155
Within academic theory, Piero Sraffa first exposed this disconnection in “The Laws of Returns under Competitive Conditions”, Economic Journal, 36 (144), 1926, pp. 535–550: in industry, unit costs are constant or decreasing over the relevant range of output, and what limits the output of a firm is the market for its products, not a rising marginal cost. Fabra sees in the article the first blow to the marginalist doctrine and judges the critique incomplete, because it did not show that the hypothesis of diminishing returns served to justify the psychological conception of value (P. Fabra, Capital for Profit, Rowman & Littlefield, 1991, p. xxii and pp. 315–316). See the appendix Objective Political Economy and Sraffa. ↩