I've found that being (potentially) the sole owner of Velvet Dyeworks (of which I own 3) resulted in a massive increase of revenue both passively and actively.
Passively, I managed to stabilize the issue with halting and going under 10,000 Capital by setting the hold rate to 50% but the income was a measly 0-50, averaging ~25 per shop with the real income being the Velvets (about 2-3 per day per shop) that could be sold for ~200 each with little trouble. As I started buying out and then converting (and then selling) the other dyeworks in Calradia, the passive income gradually skyrocketed to the present time of averaging 200 gold per day with a range of ~150-300, sometimes shooting into 400+ territory. The cost of buying out/selling dyeworks became increasingly trivial in light of the income made both passively and actively.
Actively, as I said, the going rate for a single unit of Velvet without looking for where it's scarce is about 200 by the time 200 days have passed and the economy more or less stabilizes, but as I bought out the competition and emptied the market of Velvet the going rate likewise skyrocketed so that ~400 is the new average and ~600 can be found in remote areas very far from any dyeworks. Considering you can make hundreds of Velvets per shop per year, the potential revenue goes from ~16,800 (84*200) to ~32,400 (84*400) at the end of this process--assuming 1 Velvet per day as a minimum. Actual revenue should be at least twice the stated numbers if not more depending on how much Velvet you collect in the Warehouse when hoarding outputs. Note that I'm not entirely certain I've monopolized the Velvet Dyeworks industry but, if I haven't, it's just one more in Aserai lands before I have since every city in Calradia proper and the remote frontiers have been checked by this point.
As it stands, unless Workshops have their passive income potential buffed it's necessary to become a monopolist if you want to make real money passively and highly encouraged to use them as means of mass-producing cheap goods for sale abroad at any rate.
Caravans have been rather simple and stable, in contrast, with an average income of 300 per day per caravan (with difficult to discern difference between Trade mattering versus Scout) with a high degree of variation per day (rarely as high as 1,000 or as low as negative 500). I haven't been paying them as much attention due to their consistency, though.