Income

Income is measured in Billions of Credits (BC).

Flat

 * Each 2 population on a planet (rounded upwards) produces 1 BC per turn passively, modified by any income from tax modifiers.
 * Tax rate converts a percentage of net production to flat BC, modified by any income from tax modifiers.
 * Each point of food surplus produces 1 BC, up to the population of the planet.

Percentage modifiers
Note that modifiers to tax income are applied separately as above.

Trade
Each trading freighter generates 2 BC per turn.

Consumption

 * Upkeep
 * Rush construction: A construction can be rushed for a cost in BC, eliminating the remaining production cost and completing it at the end of the turn. The efficiency depends on the amount of progress already accumulated on the item, from 4 BC per production at 0% progress to 2 BC per production at 50% or more progress.