well, merchants are definitely worth it. In my current Milan campaign, they are earning around 5k per turn, which is almost the same as my best city at this point! I have them stationed on the gold & ivory in timbuktu, spices in aleppo(one guy there makes 900 alone) and the silk in constantinople. It'll rise soon, as I've just positioned the merchants in timbuktu, and they'll soon get the monopolist skill....

Oh, and no, I'm not using the fort exploit...