Money Supply and Prices
As with any metal, those used in coinage (gold, silver, and copper) must be produced, and the tasks of mining, smelting, and minting constitute their costs of production. In exchange, unformed metal is valued for the qualities it lends to items that may be fashioned from it, but more typically it is valued by its rarity. If silver were more plentiful than iron, then coin would be made of iron and nails made of silver.
As with any commodity, its scarcity determines its value in exchange. A miner might be paid his wages in bits of iron or bits of solver, and his chief concern is having sufficient wages, whatever the substance, that can be exchanged in the market for goods necessary to the sustenance of his household.
Suppose that the amount of hard money in a market were doubled, by the introduction of metal from foreign markets. As the metal becomes less scarce, it becomes less valued and more of it is demanded in trade. That is to say that prices and wages seem to rise, but this is merely because the metal in which they are paid is less valuable than it was before.
Specifically, the amount of goods that are wanted do not double if the amount of money doubles. And as it the goods that are wanted, with money being merely the token for which goods are exchanged, the amount of money is of little concern to the welfare and prosperity of a people.
Cantillon goes further to draw comparisons to other goods, such as food (grain or cattle). In a direct barter system, the ratio of exchange depends on scarcity and plentitude: if there is a bountiful harvest of wheat, more of it will be demanded in exchange of meat; but if there is a poor harvest and wheat becomes dear, less of it will be demanded.
The same is true of money: the more of it that exists in a market, the more is demanded to obtain another good. It is not that the value of the good has increased, but that the value of money has decreased.
And as with other goods, when there is a plentitude in one market and a scarcity in another, the good is then exported because it commands more in exchange in markets in which it is scarce - provided the difference does not exceed the cost of transportation.