I have tried this and it seem to work for single units and spells and prevents rampant negative values on AI's. This can hamper an already restrained AI but it keeps the game fair. For all spells and resources that "cost" something simply put that something as a prereq. This makes it so that the game will not cast a spell unless it has X amount of mana. This also prevents negative horses and iron as I have seen on the AI screen.