It works something like that, with modifiers of course, and I'm not sure of the exact numbers. The amount of revenue a colony generates depends on two things: the "tax paying" population of the colony (which is the reported population, the game doesn't report those that aren't paying taxes), and the economy bonus that applies to that colony.
The economy bonuses can come from racial abilities, technologies, economy resources, and improvements built on the planet. Improvements that will boost the taxes consist of Market Centers, Adv. Market Centers, Trade Centers, Banks, Galactic Stock Exchanges, and the Economic Capitol.
All bonuses are additive and apply to the base income, not the current income, so if you've got a planet that is bringing in 20BC per turn, and you build 5 Trade Centers (a 15% econ bonus), each center adds 3BC per turn (resulting in 35BC per turn) rather than multiplying the existing amount by 1.15 (which would result in 37BC, or 40BC if there were no rounding until after the last improvement was calculated).
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