I've seen the other thread on "planetary economics" that has some formulas for cargo demand at planets.
A few more questions on cargo and pricing.
Does the size/atmosphere/hydrographics of the source and sale planet influence cargo pricing when I sell? (i.e. is all Terraformed cargo the same and all COlony cargo the same)
Does the price vary if I undersell the max? I know that if I oversell, price goes down.