Trade income is a little counterintuitive.
Each trade good has a Price, which is influenced by Supply (how much of the good is being produced throughout the world; it's mostly only affected by whether the province is looted/blockaded or not) and Demand, which is unique for each good (for example, naval supplies are a lot more valuable if a war involving lots of big ships is happening, and grain is more valuable when more army buildings exist, among other things). You can mouse over a trade good's price in the province infocard to see all the supply/demand modifiers for that good.
Each province has a number of production Units, which are influenced by population and base tax (units = min(2, pop/100000 + 0.995) + base_tax/20).
A province's Trade Value is given by:
trade_value = (Units*Price + additive_modifiers) * (multiplicative_modifiers + 100%)
Where additive_modifiers is any additions to trade value (the only one that comes to mind is a Trade Depot) and multiplicative_modifiers is stuff like the Prestige and Tropical global trade modifiers, the modifiers from Marketplaces and the like, and any Global Trade Income Modifiers (sound tolls, various decisions, east indian trade route, post offices, etc).
Trade Value can be seen in any province's infocard directly underneath the trade good's Price and, unlike Price, is specific to that province. Trade Value affects:
- A CoT's value is the sum of the trade value of all provinces trading through that CoT (including itself).
- A province's production income (with the exception of Gold, which behaves differently) is given by production = trade_value * production_efficiency.