In my experience, early game you want enough gold per turn to maybe buy an early unit (a settler or builder) to speed up your early game, and have enough money to pay for a moderate army. If you're conquering early, its ok to straddle the line at 0 or even -3 or so gpt, the rewards are worth it.
Once you reach midieval era units, you're going to want to have a few more traders out in order to pay for a better army. You don't need a million gpt, but you want enough to pay for unit upgrades, unit maintenance, and the occassional building.
Late game you should be getting a fair amount of gpt, I've gone from anywhere from 200 to 1000, depending on my civ and setup (england can get a lot, for instance), and late game civics can give you nice bonuses for this.