Maybe introduce bonuses for each race and a ever increasing price eg first 10 set price next 10 slightly higher so on and so forth of siege equipment. warring races such as orcs may be better having a lower starting price for siege where by they can purchase more faster eg first 20 set then nest 20 set. And humans have a faster builld time being more techniqually minded but have to pay a slightly higher cost also.
That i belive would bring a good edge to the game and alllow the siege to work in diffrent ways for each race. Letting the players work on there races particular race. Also would let there be a good ammount of siege but once the price becomes to much. Troops would take a higher precidance retaining the balance of the siege engine armyies of other games that are indestructable.