“Moreover, money is not a standard for the measurement of prices; it is a medium whose exchange ratio varies in the same way, although as a rule not with the same speed and to the same extent, in which the mutual exchange ratios of the vendible commodities and services vary.”
a few sentences below he states that
However, as they speak of prices and costs in general and confront prices and costs, they tacitly imply the existence and the use of money. Prices are always money prices, and costs cannot be taken into account in economic calculation if not expressed in terms of money. If one does not resort to terms of money, costs are expressed in complex quantities of diverse goods and services to be expended for the procurement of a product.
The price of something for an individual is not its nominal amount, but the opportunity cost associated with that amount. What he means is $5 can be more to one person than to another, thus changes in opportunity costs lead to changes in prices. That prices for a good tend to fall as its supply increases is merely a loose macroscopic tendency of this individual phenomenon. Thus, a strict equation relating changes in supply and price is impossible, much to the frustration of the mathematical economists.
Rothbard expresses more clearly the point Mises is making here:
“Money does not “measure” prices or values; it is the common denominator for their expression. In short, prices are expressed in money; they are not measured by it.”
Or you could go with Lilburne. My explanation takes the surrounding context too much into account.
I’ll take a stab at it.
You can measure length in inches, and weight in pounds. But you can’t measure prices in money. Because the inch and the pound don’t shrink and expand as time goes on, but money does. Because just as the price of strawberries can go up and down, based on changes in supply and demand for strawberries, so too the “price” [=what you have to give up to get some] of money goes up and down, based on changing supplies and demands for money.