Information is from here .
Tanks are 4x tougher than troops, but cost only 3.5x more in upkeep and 2.25x more food. Spies cost 5.3x more in upkeep and consume 3.75x more food. Either have just enough to defend or tons to be on the offensive with espionage, probably not worth being in the middle somewhere. I would guess that has changed however. But for residences each one can house 3 times more people than an acre of land.