I once had a similar Idea. But instead of making food and wood dependent on market prices, I bound Gold on the food/wood market prices. So the gold trickle would decrease.
So the more food/wood you would need to sell for gold, the less gold you would also get from the feitoria.
But recently I made also a mathematical approach, where I took even more properties of the feitoria into account. With this theorycraft it would be quite easy for all players to estimate the economic power of the feitoria at all stages of the game.