You're still supporting the slave trade, and the price point is going to be set to enable the slave trade to acquire more slaves in the future. What you want to do is buy the slaves, then follow the slaver to his next meeting with the slave catchers, and kill every non-slave there and take all their stuff. Unfortunately, we are under a time and 'don't get the Expedition in trouble' crunch here, so at absolute best we might rob the slaver on the way out, preferably with the aid of Ranald's blessing to let us run into him outside the city gates.