How to attain a monopoly...

[Daveybaby]

Actually, the monopolist trait series gives up to +3 finance skill.

Find a region with 2 of the same resource on it. Put a merchant on 1 (or both) of the resources. After a number of turns your merchant's skill will go up, as follows:

From 5 to 9 turns -> Skill +1 (Capitalist)
From 10 to 19 turns -> Skill +2 (Market Controller)
20 or more turns -> Skill +3 (Monopolist)

The actual direct cash gains you get from monopolising a resource seem to be fairly minor. The indirect financial gains you get from having higher skill merchants are much more significant IMO - their finance skill is used as a multiplier for whatever they are trading, plus higher skill merchants are better for taking out enemy merchants with.

[gardibolt]

Right, having a monopoly gets the relevant merchants the Monopolist trait, which increases their skills and indirectly raises money, but it doesn't directly generate more money off the resource itself.