For basic stuff (grain, fish, salt, cloth, maybe wine) population numbers minus what's already produced in the econ area. Similar would apply for luxury items (tea, sugar, spices) but the number of consumers be modified by some factor of technology / innovation and time - just a way of determing the numbers of 'polite / sophisticated society with cash to spare for such good. Generally once introduced to an area the demand for these goods will slowly spread throughout society, cunning traders can then introduce an item to a new market knowing that the area doesn't produce any and they can control the supply and price making huge profits but probably enemies at the same time. Also if there isn't any of that particular product in the home CoT or neighbouring ones, then there shouldn't be any demand for it - if they don't know about it then they won't want it!
Perhaps for military items (metals, naval stores, and cloth) these would also vary depending on the size of the armed forces, this could be a more realistic way of restricting the size of armed forces as they can increase the demand of an item hugely thus massively raising maintenance costs. Building new armies and particularly navies should increase demand for these items as well.
I'm thinking on my feet here, but the more I go into it, the more millage there seems to be it, although it might be too detailed for some players, it would give a whole new aspect to trading nations.