Honestly when developing an economy, it depends on how advanced it is. But in general I find starting off with something similar to the Japanese Koku is the best choice.
The Koku being a unit of measurement representing "How much rice it would take to feed one person for one year" which is roughly 5 bushels of rice, or 1000 "rice cups" (gou) or about 750 cups of rice.
Which from there you would need to determine how much rice is produced. For purposes of this discussion lets say each acre produces 5000 lbs of rice (not as much as modern standards which is about 7000 lbs), or 12500 cups of rice approximately. Which means each acre of land dedicated to rice farming would produce about 16 2/3 Koku of "wealth"
For what would make one a noble wealth wise, historically to be a "Daimyo" you needed a tax income of 10,000 Koku per year. Which would take about 600 acres of land on its own, not counting the cost of labor. Which will likely be 3-4 people per acre of land. Which will take around 190 additional acres of land on the high end. Or for quicker calculations, assume 3/4 of land can be counted as "taxable" land while the remaining 1/4 is labor cost, assuming a more feudalistic serf system of course.
Of course there are other factor such as percentage of crops lost, and it can be fairly said that anywhere between 10-50% of the crops can be lost without significant care and technology to prevent foraging from animals, plant health, and the like. So that'll increase the land usage. And that would be on an average year of course, bad years can make it even worse.
And for the land of a noble, don't assume all land is usable as farming land. Using averages based on our technology today, only about 35-40% of all land is farmable, and this number can easily drop depending on region and available technology. So that base level of 800 acres of land being needed would represent only about 2012-2300 acres of land a noble would need.
Could go into more details here, but I think getting any more detailed would be useless for your average rp or rpg setup.
But from there, you simple price how much the Koku is worth and how much a cup of rice is worth and you've got the "money" base units to work with and can expand out further.
Your support makes Blue Moon possible (Patreon)
