  Starting from a plausible assumption about the Total Revenue concept, a
system of economic agents, that simulates the exchange of goods, is studied.
Following a methodology equivalent to that used in the statistical-mechanical
determination of the distribution of energies in a physical system, it is shown
that the price of the exchanged goods arises naturally, as well as the
well-known ``Law of demand". It is also shown that by using Euler's Theorem of
homogeneous functions, the conditions emerge for the appearance of wholesale
and retail prices. We found also that the Total Revenue is the product of two
factors: the configurational entropy and a "free energy" type one. We study
numerically the case in which the exchanged good is money itself, in systems in
which economic agents have a "roof" on the amount of money they can possess. It
is shown numerically that the increase in the quantity of money in the hands of
the agents necessarily leads to its depreciation, and only the expansion of the
economy, understood here as raising the "roof", minimizes this loss of
purchasing power. The concept of indexing is discussed.
