A settlement seems to make more money if it can trade with other settlements that do not posses the resources it does.
For example if settlements A, B, and C all border each other and only have coal to trade they won't make much money because they're trading 'common goods'. But if A has coal, B has fish, and C has furs they'll make more money through trade since they each possess a resource their neighbour lacks.
This can also be used to make wars more expensive because if Faction A and Faction B are trading resources only they possess with each other they'll both lose more money through lost trade than if they were trading the same resources.
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