Would you be happier if I said that market prices in a competitive economy (that is, "exchange value") are the values of the co-state variables that appear in the Lagrangian for the economic problem of arranging production and distribution so as to maximize the chosen welfare function?*
*Where the welfare function is "chosen" by the society's
distribution of income and wealth: the greater your income
and wealth, the greater weight your preferences have in
determining the shape of the welfare function that the
competitive market economy tries to maximize. For this
reason most--liberal--economists think that if you take
care of the distribution of income and wealth (and
externalities, and monopoly) that most other economic
good things will take care of themselves.)
Market prices in a competitive economy ("exchange values") *are* indicators of scarcity: they carry information about the money-metric utility of that particular commodity or resource in its most favorable alternative use. This is one of the two key reasons that markets appear to be very flexible and "efficient" mechanisms for allocating production and distribution (but if the distribution of wealth is wrong, or if externalities or monopoly are rife, then they will flexibly and efficiently carry you to the wrong destination).
The second key reason is that the possibility of bankruptcy appears better than alternatives in enforcing "sunset" on inefficient and unproductive organizations. Certainly neither bureaucracy nor politics are very good at closing down organizations that have outlived their usefullness. As former Undersecretary of Commerce Ev Ehrlich once said, "It is a good thing we didn't have the Federal Government around 3,000 years ago. We would still have 4,000 people down on the Mall working in the Department of Hunting and Gathering."
But I digress...
Brad DeLong