We use things like region type, location, presence and type of coastline, percentage of coastline, and I think a couple other things, to allocate population to different sectors of the economy. This determines what percentage of the population does each different thing, such as mining, farming, fishing, hunting, manufacturing, trade, etc. This is modified by quality of that sector for that type. I.e. you can farm in a badlands, but it's not very effective. Fishing in the ocean is great, and better than fishing on a river. The food and gold income from each sector is then calculated. So you can get both food and gold from, say, hunting. But mining only brings in gold. Some of these are further modified by other things, such as population density. Adding together the food and gold produced by all the sectors gives you the final food and gold values for the region.
If we ever do introduce other goods, such as wood and stone, this gives us an easy way to determine which regions produce how much of each, as we already know how many peasants work each sector, and how good that sector is in that region.
