So, your PCs have grown in power and influence, and they want to start their own Slugman house, or they have taken control of one etc. They wonder about the budget. How do the numbers look?
Pretty big. So big in fact that we need a new unit of currency to make the numbers manageable. For Yoon Suin, I propose the gold talent, which corresponds to 26 kg (!) of gold.
(Because I'm a stickler for details in my game a talent is worth 6771 gp. I recommend a more manageable number if you want to do this yourself, but I'm doing the work for you so you don't have to suffer)
The 5e PHB tells us that the aristocratic lifestyle costs 10gp+ per day - this includes fancy dwelling, fine foods and clothing, servants, guards etc... so essentially, you can multiply 10 X # slugmen X 365 to get the yearly cost of running a slugman house - this isn't even any "special projects", this is just keeping your slugmen fed, housed, and served upon. A medium sized (100 members) house has therefore the *alarming* expenses of about 54 talent a year (ie 365 000 gp)! If we round this up a bit, we end up with 0.5 gold talents/year/slugman, which is a huge value *but* facilitates calculations. So the House of Leaves, with 56 members, requires at least 28 gold talents a year to break even.
The easiest income source for slugmen is rent collection, as they own all buildings in the Yellow City. - in fact, rent is the main form of "taxation". Using a modification of these numbers (185 gp yearly income for non-slugmen citizens), an average size of 4500 people in an average wealth district, 40% rent (but only 30% collected - the missing 10% is for expenses, maintenance of buildings etc), you get about 37 gold talents a year. This is fine if you are a small house, but what if you're the house of White and Black with 200 members? You need other business ventures.
Here are a few examples.
Pottery smuggling ring:
1 ship carrying 200 tons dropped off on an island, 4 shipment a year. 1 item of pottery = 1 gp. 1 item = 1 pound but packaging etc, so 1 ton = 500 items = 500 gp. Duty is 30%, you avoid it but you have expenses so your profit on the smuggling is 15%. Total about 9 talents a year.
100 river boat loads a year. Based on my research, that's about 500 tons a year (this is a realistic number. I figure that the Yellow city consumes at least a ton a day, so add slugmen excess, exports to Xian etc...). 1 tea brick =4 rupees and slightly over a pound, or about 112 talents. Slap your monopolistic 30% duty on it, and that's 34 talents. This is why the Council House are so wealthy.
For that we can thank Pliny the Elder. If expenses are included already in that price, we are at about 3 gold talents a year, not that great (edit: previous calculations were wrong), but this is in net profit, and if you slap a few duties on export, you can probably squeeze a few more talents. The main weakness in using Pliny is the lack of knowledge about how well his prices line up up with Yoon-Suin prices. The scale is interesting here. 3 talents a year is a lot of money, and yet it's barely sufficient to sustain 6 slugmen!
Hair transplant services.
You laugh, but my players *actually did this*. At its peak, the business was pulling about 3 talents a year, which is really good for a small shop offering slugmen of discerning taste the unique experience of having hair. This is not sustainable (fads end) and relied on having a unique artefact, but it shows how a clever adventurer can get *much more* out of a quirky item than by simply auctioning it off...
Lastly, I will note that the GDP of the Yellow City would be about 10 000 talents a year *were it a city of average wealth* - as we know it's a wealthy place due to its position as a the sole port of entry for the entirety of Yoon-Suin, it could be triple that. The slugmen, at 3% of the total population, consume 1/6 of the resources...