  The distribution of wealth among the members of a society is herein assumed
to result from two fundamental mechanisms, trade and investment. An empirical
distribution of wealth shows an abrupt change between the low-medium range,
that may be fitted by a non-monotonic function with an exponential-like tail
such as a Gamma distribution, and the high wealth range, that is well fitted by
a Pareto or inverse power-law function. We demonstrate that an appropriate
trade-investment model, depending on three adjustable parameters associated
with the total wealth of a society, a social differentiation among agents, and
economic volatility referred to as investment can successfully reproduce the
distribution of empirical wealth data in the low, medium and high ranges.
Finally, we provide an economic interpretation of the mechanisms in the model
and, in particular, we discuss the difference between Classical and
Neoclassical theories regarding the concepts of {\it value} and {\it price}. We
consider the importance that out-of-equilibrium trade transactions, where the
prices differ from values, have in real economic societies.
