I don't think this kind of play needs an accurate implementation of economic formula, which I'm pretty sure is not what Roger meant, anyways. But I also don't think we need that detailed of an economic system. We certainly don't need much in the way of an "economic combat" system.
The goal as I see it is to determine whether the community is growing and whether it can support certain defenses: dedicated law enforcement, medical services, emergency supplies, protection against catastrophes. The more general services -- entertainment, food, supplies, communication, and exportable resources -- a community has, the more reason people have to move there and stay there. The larger the community, the more it can afford in terms of defenses, which in turn add to the attractiveness of the community, causing it to grow more.
On the other hand, each feature of a town and its surrounding area has an innate risk. For example, a saloon with liquor and gambling is an attraction for the miners or ranchers in the area, but carries with it the risks of drunken brawls and violent disputes over honesty.
The way I'm imagining this potential system is that you have a list of goods and services that could be applied at different scales, so that beginning characters could focus on establishing a saloon, a bank, or a ranch, selecting what improvements they invest in (similar to the stronghold investments in Book 3 of the LBBs.) The scale rating is just basically how many customers an establishment supports, so many of the improvements could be scaled up to apply to the entire town, for more experienced players who are acting as leading citizens. Each improvement would also list its associated potential risks.
The GM determines the base growth rate based on the investments made. This would be a very loose calculation; maybe something like a flat 10% increase every "town turn". Every turn, the GM makes two rolls:
- A reaction roll for potential immigrants, to see if the the people keep coming or increase their rate of arrival, modified by improvements;
- A situation roll to see if something bad happens (fire, large brawls, stampedes, bandits.)
The reason I would develop a town system this way is to keep the "nerd burden" light. Not everyone likes to play the "Traveller trade routes" game, which is what the frontier town model has a potential to turn into if too much emphasis is placed on economics. It's easier to let players who decide to be frontier entrepreneurs pick what they're going to sell, roll up a discount for the base price, and let them pick a markup over base price to set their potential profit; future actions (finding a safer source for goods, for example) may allow a chance for a better discount, but for the most part, the players don't have to worry about it if they don't want to. The focus is on what kind of events happen as a result of their activities, not the monetary details.