It is complicated to make an artificial economic system, the only way is to make it dynamical and self-sustainable.
Implement demand and saturation curves, let the plugin count daily purchases and sells on the bases, ratio of raw to refined goods and adjust the prices every time server restarts.
To control the frequency and amount of adjustment use some coefficients that will set the dynamics of market (like, - d(cr)/d(good) ) ... For instance +1 credit price increase for every -500 goods taken.
