Let's participate in an exploratory exercise in order to produce a coherent, convenient micro-economic framework for your world. You may or may not know that I have a macro-economic system that I use ... let's leave that by the by, however, and steal from another system entirely, one that is familiar: Civilization IV.
(sorry, I'm not familiar with Civ V; I heard it was shit and never bought the game - so you're stuck with me talking about IV)
Typically, you start Civilization with a village that produces food, hammers and coin. This village is, in the game, recognized in size to be equal to 1 ... and this 1 requires two food to feed it. Luckily, it is also assumed that your new village controls one square of hinterland, and this provides you with surplus food (as well as hammers and coin), and so your village can grow and get bigger. When it grows to size 2, it requires four food to feed it, but that's okay because now you control even more hinterland - and so on.
Since, in the game, you are a thriving city by size 4, we must assume that the food required (and thus produced) increases at a geometric rate. The game doesn't care about this, of course - but let's care ourselves, because we want to understand this progression in terms of D&D.
Let's set a size of 1 as equal to a village, and let's say that village has 500 people. Further, in keeping with the geometric progression of increased numbers, let's translate "two food" as equal to '11' food ... understanding that the 11 is a binary number. This means that the first digit represents 2 to the first power (2) and that the second digit is equal to the power of zero (1). Thus, the "2" in Civilization = binary 11, or can be translated as a "3" in the ten-based system we're familiar with.
I hope this is clear, and that I haven't lost half the audience. If you're not familiar with binary numbers, you might want to start here, read for a bit and then come back.
Therefore, our village of 500 requires 3 food to survive. Using the minimal survival quotient (and I'll use that defined by the U.S. Army to feed Berlin during the Airlift in 1948, since it's hard to argue) of 1,700 calories per person per day, our village requires 310,250,000 calories per year. We can thus define each food required as being equal to 100 million calories (letting our medieval villager starve a little). Just to emphasize - we're not talking the "2 food" referenced in Civ IV, but its translation into our calculations, which means 3 food. When I refer to Civ IV numbers from here on, I'll note them in () ... thus, 2 food would be described as (2), which equals in binary 11, or 3 in my system. Clear yet?
Now, in Civ IV, a prime piece of land, a plain, also produces (2) food ... and if you add a farm, it produces (3) food. For us, the latter equals 7 food, or by our reckoning, 700,000,000 calories. Thus, our village positioned next to a nice hinterland plain has the potential to produce 1 billion calories altogether - enough to feed 1,666 people. This is a good thing, since a laborer really needs much more than 1,700 calories a day - 3,000 to 4,000 calories is more the requirement for hard labor ... but then, the laborer only needs that during the heaviest working periods, and obviously the children and the old don't need that much. Still, a billion calories is enough for our village of 500 to eat very hearty ... and its enough to draw other people to our village, where the food is plentiful.
According to Civ IV, it would take 11 turns for our village to grow to size (2) ... which in our ten-based system would be equal to 3 x 500 people (isn't it nice when the math works out). For the city to grow in Civ IV from size (1) to size (2), I'm guessing it requires an accumulation of 33 additional food (I've never been quite clear about the exact number needed for each city size).
These 11 turns, depending on the time of the game, can be anywhere from 220 years to 11 years ... I think for my world, we want a renaissance speed. I'm afraid I can't remember what epic is around 1650 ... I haven't played epic for awhile, and I don't have the game in front of me as I write this. If someone wants to jump on me, they should ... but in the meantime I'm going to call a "turn" two years.
Thus, our village is gaining, on average, 53 new citizens per year.
If, then, your party enters into an area like this as landholders, and they have a village with a set size which they wish to grow, what they need is to accumulate enough food to increase their village's growth. This could be done by building a farm where there was no previous farm, or introducing fishing boats into a village where there were no previous fishing boats ... or simply plundering food from outside their immediate system equal to the necessary calories required to raise the size of their village. Naturally, they could also seize persons as slaves/followers, and here you now have a system to determine how much food those slaves/followers could produce in another section of hinterland, using the Civ IV measures for how much food areas like desert, oasis, forests, plains, sea coasts and so forth produce. This will enable you to measure your entire food production right down to the last calorie, if need be, while gauging your micro-economy's growth in a rational, predictable manner.
More to the point, you can introduce a randomness to this (and I might in a few posts, there are other things I want to cover first) that will break down the calorie producing capacity of regions. Rather than using one food = 100 million, you could say 1 food = 10 million, and then designate a plain as producing 6 to 36 food rather than (2) as it is listed in Civ IV. That is really up to your interest in playing with the odds.
I'll be tackling coin next ... but I would encourage discussion of the above, since it will give me an idea of what problems I need to work out regarding how coin would work.