In my unprofessional opinion you need to develop a base cost for all materials that are to be traded. You then need to create a rating system to determine the base scarcity or abundance of the material for each province. Next would be the time scale for change either monthly or seasonal. Then you would have to create a chart for the events that could occur and the modifiers that they could apply to the product war, poor harvest, boom etc. Further more you would have to determine a base population for the provinces and the modifiers that could occur due to events plagues , war, economic expansion.

you obviously know all of this but start with the baseline and then go from there. If you do not have the base then the rest seems overwhelming in my opinion. Unless your players are really into this then it really have no value besides an intellectual exercise. Good luck

The other method that i have seen as functional was used to the darksun traders book that left the values fixed until the players arrived at a location and then the random factor was determined this simulated the effects of random actions upon arrival not predetermined values that do not directly affect the characters. But it is your world .