Ditto.
Also, I wish to put in my two cents and disagree with myself for earlier.
In the name of enshrining neoliberalism into the game, I think you shouldn't count trade balances. They don't matter anyways, as capital balances and the free flow of gold will wholly adjust them. Ultimately all total net trade balances are and must be transient.
Don't listen to the mercantilo-racists who tell you otherwise!
Basically what I have it down to now, via the JPierreD idea of having broken down stats for each region/colony of an Empire, is that the GM will have a bit of liberty in deciding what the the income level of each region/colony should be and, all added up, will equal your total income.
So, Caribbean sugar islands will boost your income. Middle Asian opium fields will boost it. The Saharan desert will not boost it very much at all. This will be the simplest and I think most game-friendly way to do it.
And, of course, things CAN be changed after the game starts if we find something doesn't work as great as we like, etc. etc.