Taxes

Urban Planets provide income in the form of taxes. The amount you received is based on your tax rate as well as your total population. There is an upkeep cost of $500 per planet to take in to account as well as the upkeep costs for anti-pollution planets (to deal with your people's pollution), food planets (to feed your people), government planets (to keep your civilian mood high), and education planets (to increase your population growth rate) which are all $500 per planet.

The formula seems to be ([Total Population] * [Tax Rate] * (.28)). (e.g. with 7.96 billion population at 31% tax rate I receive 764,858,217 income. This number is offset by upkeep costs. However, in this example the total upkeep cost for anti-pollution, food, government, and education planets is only $58,972,000 which leaves me with a total of $705,886,217 (some of this upkeep cost is actually extra because the anti-pollution and food planets support petroleum planets and troops, respectively). This example is of 42,530 Urban Planets which gives a return of $16,597 per planet!