Outside of Birthright, I've never read anything that's meant for domain level play. I see Bruce Heard's breakdowns of the Mystaran provinces every week, and it's been enough finally to get me into gear to do some economics and demographics. I spent a while looking at the Domesday Book as well as some other sources to come up with these calculations, but most of it is done on "should bes" rather than solid research. At first I was embarrassed to have so little solid information to base this on, but then I remembered I'm not writing a paper for a conference, I'm just trying to work some things out for my game—besides which, it has to jive with PHB and DMG prices, which means it's probably not going to be accurate anyway.
Here is my rough math, and I'm GLAD to hear anyone's corrections:
Based on the incomes of several estates (net incomes, now, so we can't parse what they were comprised of) I have assessed that a wealthy estate could pull in around £170 per annum. This was primarily in goods as a result of labor on the lord's demesne. A pound sterling being simply a pound of silver and 100 silver coins being a pound in AD&D leads me to assess this at 17,000sp/annum for a small but wealthy village of 20-25 households. To make it easy, we can just round that number out to 20,000sp for a wealthy estate, 15,000sp for a moderately run estate, and 7,000sp for a poor one.
Assuming that the lord has just the right amount of land to be worked by his men (we'll say 5 hides for 20 households, blithely assuming that a single household can work a whole hide with the standard 2-3 days of labor owed per week), we can assess the value of goods per house produced by the settlement: 750sp/household/annum. Some further math will yield a price of 2sp for a bushel of wheat at this rate (based on a production value of 1200bu/grain/harvest for a hide).
I've gone on further to assume that 3/4ths of the value of the estate's income is in goods, 1/4th in fees and rents from the year. With the flat rule that another 1/4th of the estate's incomes can be sold or traded, this evenly divides the net annual income of the estate into halves: one half in food (ale, butter, apples, peas, grains, etc.) and one have in cash receipts. A town of 20 households (approximately 80 people) produces 7,500 silver (750) in food and 7,500 silver in cash for its lord.
Now, most lords would hold a manor of 3-6 towns plus potentially a central village. We'll say the village has a population of some 1,200 people, which is between 300-400 households. Given the standard "fertile but not great" value of 75gp/annum, we arrive at 22,500gp for 300 households. This, plus the 1,500gp from each of the small towns (let's say 3) leaves our lord with a net income of 27,000gp. A fair amount! 6,750gp of this is taken in fees and other cash incomes. A further 6,750 (for a total of 13,500) can be sold or traded for cash receipts.
That means the lord can easily buy 540 swords a year if he's looking to outfit an army. Knights and other household can be "paid for" from the food-income side of his earnings, since they don't require a stipend. Skilled labor, such as a master mason or mercenary soldiers, must still be paid from the cash side of the income (though they may also take up food and residence!).
Most of the lord's upkeep is free. since it comes from free labor services. That is, unless he wants to build something from stone which requires that he find the resource (perhaps he has a quarry, but more likely he'll have to pay for it) as well as the stonemasons to oversee the labor of the peasants who are being taken off their work. This can reduce the incomes of the lord as now the common folk are repairing bridges or building a castle instead of working his land. I think it's fair to subtract a certain value from his incomes for this.
Looking at a lot of the numbers, I think this rounds out pretty nicely. I'm going to experiment with it in play, as well as work in some randomness and the like. This also fails to account for the imperial tax levied unevenly upon nobles (which can be up to a third of the value of their lands).
Anyway, a good comparison is thus: a suit of plate armor costs 10,000ƒ (fulcre, pillars) in the empire due to the rarity of pure enough steel and the secret of its craftsmanship, which means only dwarves can do the smelting for it. This would be a quarter or so of our fictitious 3-village 1-town lord's income for the year: fair, I think!