[Michael Cule]

Well, it entirely depends on the amount of handwavium you want in your game.

I don't know that there's a single system that will tell you what you need to know. Even establishing how much labour, land and seed goes into getting your community fed varies enormously by tech level, locations, culture and half a dozen other factors. HARNMANOR is a good look at doing it in a detailed way for a medieval setting.

If you were to extend using GURPS to the higher level game you would want the leaders of your community to make rolls (on a seasonal? yearly? basis) on at least the following skills:

Administration
Agronomy
Diplomacy
Leadership
Merchant
Politics

Each of these areas (and I'm sure there are some I've missed) has potential going for very well or very badly.

And that doesn't tell you how much of your wealth you should spend on military production or how much of your time and personnel you should divert to luxury goods you can trade.

If you don't want to try to build a system then you might look at REIGN which is a game designed to run campaigns of 'lords and leaders'. The company rules from there could be used as a cap system with something like GURPS providing the 'agents and operatives' level. Very full of handwavium though.