Contrary to the theory of the subjectivists, and neoclassical economists, therefore, it is not the consumer/demand that determines value/price on the basis that something is only ever worth what someone is prepared to pay for it. Rather, the consumer must accept, for any commodity they demand, the price that the seller is asking, and that price is, also, not arbitrary, but determined by its value/price of production. The buyer may, of course, on that basis, decide that, at that price, it does not represent use-value for them. They withdraw their demand for it. The consequence of this withdrawal of demand can only be to reduce the level of supply, not to determine the value/price of the commodity.
The reduction of demand, will cause, in the short-term, an excess of supply over demand, and consequent fall in market price, to clear the excess. Mill, Say and Ricardo did not deny the possibility of such overproduction of some commodities, but denied it could be the case for all commodities simultaneously. But, they failed to recognise that money (as opposed to currency/money tokens) is also a commodity. It is a requirement of money that it be a commodity, i.e. have both a use-value, and a value, for it to become money in the first place, which is why money tokens/currency, or things like Bitcoin, are not money. Money is the general commodity, but, as such, in acting as money, rather than as a commodity in its own right, it must give up its role as commodity. To be used as money, gold (not all gold, but only that acting as money, i.e. universal equivalent) has to give up its use-value as commodity, for example its use in jewellery.
Gold as money, as the general commodity, can be demanded as money, but then, unlike other commodities is not consumed, or necessarily exchanged. It can be hoarded or held on to. So, Marx points out an overproduction of all commodities, can, occur simultaneously, other than for money itself.
He notes,
“At a given moment, the supply of all commodities can be greater than the demand for all commodities, since the demand for the general commodity, money, exchange-value, is greater than the demand for all particular commodities, in other words the motive to turn the commodity into money, to realise its exchange-value, prevails over the motive to transform the commodity again into use-value.”
Because subjectivists, and neoclassical economics make no distinction between market price, and market value, they interpret this fall in market price, as a fall in the value of the commodity, induced by the subjective valuation of it by the consumer. But, then, some of those marginal producers, who were barely making any margin of profit, sell at a loss. Some go bust, and their supply disappears, itself, helping to clear the market. Others will reduce their supply by moving to the production of some other commodity, others by simply closing down some of their business.
The result, therefore, is not a change in the value/price of the commodity, but, simply, a reduction in its supply, and the extent of that new level will still be determined by the market value of the commodity, i.e. the level of supply that can all be sold at that market value, and so produce the average annual rate of profit. However, the market value, as set out earlier, is itself an average of the individual values of all the different producers of that commodity. If the reduced level of demand, results in reduced supply, brought about by the failure of the least efficient producers (who have the highest individual values for their production), then this also means a lower market value.
On that basis, a a lower market value/price of the commodity is consistent with producers, in aggregate, making the average annual rate of profit, on the overall reduced level of supply. To put that in context, if all the firms in this industry operated at the same level of efficiency/rate of profit, the reduction in supply, brought about by whatever means, would have no such effect on the market value, and so price. It would result only in a reduced supply, with the price remaining the same. Indeed, if all firms stayed in production, but, each, simply reduced their output, they would all lose some of the benefits of economies of scale, and so the social cost of production, for each of them would rise, resulting not in a fall in market value/price, but a rise, at this lower level of demand and supply.
In other words, a full understanding requires an understanding of value and use-value, and their role in determining supply and demand, but, then, also, requires an understanding of the whole of the interaction of supply and demand to form market prices, and the role of demand in determining the level of supply, as well as the level of supply/scale of production, and so, to value. As Marx puts it in Capital III, Chapter 10,
“Supply and demand determine the market-price, and so does the market-price, and the market-value in the further analysis, determine supply and demand. This is obvious in the case of demand, since it moves in a direction opposite to prices, swelling when prices fall, and vice versa. But this is also true of supply. Because the prices of means of production incorporated in the offered commodities determine the demand for these means of production, and thus the supply of commodities whose supply embraces the demand for these means of production. The prices of cotton are determinants in the supply of cotton goods.
To this confusion — determining prices through demand and supply, and, at the same time, determining supply and demand through prices — must be added that demand determines supply, just as supply determines demand, and production determines the market, as well as the market determines production.”