The amount of gold held by the mine differs according to the Age it is cast in. In the Archaic Age, gold mines yield 500 gold. Classical Age gold mines yield 1000 gold. The Heroic Age gold mines yield 3000 gold while the Mythic Age gold mines yield 6000 gold. Because of this it could be better in some games to keep this power until Mythic age is reached when most gold mines are gone, but it also could be helpful to get an economic edge in the early game.