Here's the formula for computing gold income in a settlement:
Gold_income = factor(pop) * mine_value + officials
- factor(pop) is the population scale factor, a logarithmic function of the settlement's population. It ranges from 0.1 to 2.0 depending on population; for a colonial city it is 1.0.
- mine_value is the mine value of the gold found in the settlement
- officials: Tax Collector +1d, Governor +1d (the additional income from officials is considered as production income when it is applied to computation of gold inflation)
The mine value of every gold province is defined in province.csv. New colonies will get that mine value. Provinces which start a scenario with a settlement may have a different mine value defined by the scenario.
Over time, events may change mine values, although there are no scripted events in vanilla EU2 which do this. (Mine values are manipulated extensively in AGCEEP.) There is also one random event New Mineral Found which adds +10 to the mine value of an existing mine.
In EU2, having too much gold income is one cause of inflation. The way this works is not known exactly, but the basic outline is this: if more than 40% of a country's monthly income is derived from gold income, then it will get a variable amount of inflation each month. The rate is always at least a rate of 0.1 points of inflation per year, when you are at exactly 40%. As the percentage of income rises, so does the rate to a maximum of 0.25 points of inflation.