"Well, it's like in the Army, you know? The great prince issues commands, founds states, vests families with fiefs -- Inferior people should not be employed."
--- Firesign Theatre
Now here's where I try to get really clever with adapting the magic research rules to all sorts of other things: barony development. Famously, Underworld & Wilderness Adventures brought up the topic of domain management as the D&D "end game", but only offered sketchy details: "Some possible investments are: Road Building, Canals, Inns, Hunting, Religion, Armories, Animal Breeding, Farming, Fishing, Exploration, Ship Building, Sea Trade, Land Trade, Trapping. Successful investments will also have the effect of increasing the population of the investor's territory, providing the area of investment does not specifically preclude such (hunting and trapping would do so, for example)." Many people have tried to flesh out those details, for example the designers of Adventurer Conqueror King, but most aim for logic and realism, working out economic details. I don't want that kind of grief, myself.
The simple way is to again define each feature in terms of levels, much like the holy shrine or the arcane library. In fact, each of those could be considered examples of barony investments. The baron spends money raised by taxes, with the intent of raising the level of the resource, and makes the research roll, with success meaning the level improves, winding up with a 1st level road network, or 3rd level trading.
For commercial resources, the level determines income. Trade, for example, brings in profits, which can be taxed for extra domain income. Since the base tax is 10 gp per citizen, you could figure that 1st level trade brings in 1d6 extra gp per merchant or tradesman. 3rd level trade brings in 3d6 gp per merchant.
The level could also be a target number for 1d20 rolls when trying to produce some specific resource. Say you invest in lumber, and suddenly need a supply of logs to build a palisade. Roll 1d20 less than or equal to your Lumber level to meet that need.