  Implementing a set of microeconomic criteria, we develop price dynamics
equations using a function of demand/supply with key symmetry properties. The
function of demand/supply can be linear or nonlinear. The type of function
determines the nature of the tail of the distribution based on the randomness
in the supply and demand. For example, if supply and demand are normally
distributed, and the function is assumed to be linear, then the density of
relative price change has behavior $x^{-2}$ for large $x$ (i.e., large
deviations). The exponent approaches $-1$ if the function of supply and demand
involves a large exponent. The falloff is exponential, i.e., $e^{-x}$, if the
function of supply and demand is logarithmic.
