As mentioned above, merchants are definitely worth it if you send them to Timbuktu. As the Spanish, you're not that far from the gold there. Only the Moors are closer, and then not by much - the biggest part of the journey is the trek across the northern Sahara. Merchants can quite happily earn several hundred/turn on the good resources there.

Ignoring the money aspect for the moment, Timbuktu is also a good place to train up your merchants. It's easy to get a monopoly on the resources there, and it's far enough away to get good traits. You could then move them back into Europe for the even-more-micromanaged part of the merchant game, aggressive takeovers of foreign traders. I usually avoid this, because I find it too tedious with low-level merchants, but with more experienced entrepreneurs it's a little more fun.