I already have 10% of production from elsewhere, so I think that I could get 33% production by taking their provinces. It sounds like I need to have 33% trading to dominate? I'm not much of a trading power...
Aren't slaves fairly worthwhile by themselves, or not?
They start out with the same value as grain or naval supplies. The demand goes up with colonization of provinces that produce cotton, tobacco and two other items. But they peak at some point and then as abolitionists come about their value plummets to where they are eventually replaced with grain.