In Rise of Legends and Rise of Nations there is a price concept called Rack-up prices. This means that every time you make a unit of a certain type, the next one you make will cost more unless the other one(s) are removed from that nation by gift or combat. Of course, Rack-up prices do stop at a certain point, so there wouldn't be a ridiculously high price. However, the unit must be purchased by you or it doesn't affect the prices.