I generally take a peaceful/expansive route in the early game, so by the time I'm ready to start seriously upgrading my military, I'm typically making 100g+ per turn (after 90% research). But if you need a quick boost to your economy for the unit upgrades, trade missions with your great merchants will go a long ways towards getting you that cash. Also, once you've built your holy shrine(s), settle your great prophets as "super specialists" in your cities; this really helps drive my economy, especially in the early-mid game.