I think of Yoon-Suin as largely an expressionistic setting, in the sense that it's not really supposed to make sense behind the scenes, but rather be evocative thematically. However, it has a veneer of realism for the players to get engaged in.
Since trade is such a big theme within the setting, I decided to come up with a simple way of making trade accessible and gameable. I tried to resist the urge to go down the route of re-jigging the Traveller speculative trade rules, because that has been done well enough already and it doesn't feel quite right in terms of what I want trade to represent. Instead, I went for something a little more complex and Yoon-Suin specific.
In Yoon-Suin, the trade goes in a big back-and-forth from region to region. The Hundred Kingdoms are fecund and fertile but resource-poor, so they produce lots of slaves and lots of animal produce. This gets sent to the Yellow City because it is the only geographical outlet, where it is shipped up the God River to the Mountains of the Moon and Sughd. Back down the river to the Yellow City from Sughd comes tea and opium, and from the Mountains of the Moon, minerals. Eventually this makes its way to the Hundred Kingdoms, which desperately need minerals in particular. The Yellow City is so important and wealthy because it is the hub of all of the trade.
So there are large regional differences to take into account as well as place-specific differences. So the system goes like this:
Goods are divided into units, each weighing 1000cn (100lb). Each type of good has a value per unit, and you obviously just tot up the value of a given cargo based on the number of units (i.e. the weight) of each cargo type. For instance, if dye is 50 gp per unit, and a merchant is carrying 2 tonnes of dye, he is carrying 40 units, and thus 2,000 gp worth of dye. (I use imperial tonnes and round them down to 2000 pounds.)
Goods have a value at source (meaning the price you pay at the place they are produced), a value where desired (meaning the price paid where the goods are not produced but not in great demand), and a value where needed (meaning the price paid where the goods are particularly in demand). The value where desired is 1.5 times the value at source, and the value where needed is 3 times the value at source. These are referred to as "Sourced", "Desired", and "Needed".
Of course, the DM can change these values for special cases. If he feels like some backwater somewhere would pay 20 times the value at Source for some special variety of tea, that's not a problem.
Slaves are always Sourced in The Hundred Kingdoms, and each polity there will also have 3 natural products which are Sourced. Other natural products in that given city will be Desired. Mineral products are Needed.
In the Yellow City, essentially everything is Desired. The city produces nothing, but it has access to all other goods in sufficient amounts that nothing is ever in huge demand. The exceptions are tea and opium, which are Needed, because of the debauchery of the city's inhabitants.
In the Oligarchies in the Mountains of the Moon, slaves and natural products are always Needed, and each oligarchy will have 3 mineral resources which are Sourced - other mineral products are Desired, as are tea and opium.
In Sughd, tea and opium are Sourced, mineral products and natural products are Desired, and slaves are Needed.
So if our merchant with his 400 units of dye, which he has bought in the Hundred Kingdoms, takes it to the Yellow City, which Desires it, he can get 1.5 times its value at source, or 3,000 gp. If he takes it to the Oligarchies, he can sell it for 6,000 gp. Once there he can buy 6,000 gp worth of, say, lapis lazuli, and sell it in the Hundred Kingdoms for 18,000 gp. Not bad work if you can get it, although you have to bear in mind that the journey is about 350 miles through dangerous and pestilent jungle, he will have to hire lots of men to keep him secure on the way, and he will likely have to pay taxes at both ends.
The rationale behind the system is that there is no need for PCs to get involved in trade, and thus no reason for the DM to bother with it either, but if it becomes relevant for whatever reason or the players want to do it, there is a rudimentary system available and ready for use. It also means that you can have goods appearing in treasure hoards and quickly work out a value based on the weight.
This system also allows for random cargo generation. When players come across a trader in a random encounter, the DM can roll a number of units and a number of cargo types and assign as necessary. Yes, if you get your shits and giggle robbing merchants and selling their cargoes elsewhere, your shits and giggles are facilitated by the Yoon-Suin rules.