This is what I've used (a lot of the terminology is specific to my campaign world, but it's easy to change)
All land in a Kingdom is owned either by the King and Queen or by one of the extended family of the King or Queen Or by various notable members of their society and their underlings as well. The Ruler grants in feudal tender parcels of land to powerful nobles called Donu(Male) or Dona(female). The Donu or Dona also grant parcels of land to other noble warriors and elite people, who have the title of Cingeth. Cingeth and their leige are roughly equal in power, wealth and responsibility, but the Donu/Dona have a higher precedence in matters of courtesy and deference. The Dona or Danu and Cingeth are obliged to provide fixed numbers of soldiers and money to their overlords based on the amount of land they posses. The Dona or Danu and Cingeth, in turn, grant parcels of land to vassals of their own.
The most common method of organizing granted land is the manor. A player will be made the equivalent of a Knight or Squire. The ruler of the manner is titled “The Master of …” such and such. A manor must be at least 1 square mile of farmable land. This will also come with 1 to 4 square miles of non-farmable land: 1 square mile if in normal agricultural area, 2-4 in heavily forested, swampy, dry or hilly country. The manor then is subdivided into “plots”. Each plot equals 25 acres and there are 25 such plots in a square mile of farmable land. Each plot supports a family with an average of 5 members. The plot produces 150 gp per year, yielding 1250 gp per square mile of farmable land.
In exchange for the grant of land, a knight must serve in his overlord’s service for 60 days per year, providing his own heavy warhorse, chain mail armor (or better), shield, lance and sword, and bringing a squire-at-arms on a light war horse and scale armor (or better) and a hand weapon. The Knight must also feed himself, his squire and horses during the service and is required to keep his equipment in serviceable condition. Usually, the overlord expects 1 knight and squire-at-arms for each square mile of farmable land.
A Squire’s Duties
A landed squire with a grant of land pays “scutage” or “shield money” in place of military service. This payment is equal to 300 gp per square mile of farmable land, and is roughly the cost for the overlord to pay and maintain a 4th level fighter for 60 days.
The Farmers’ Duties
Each 25-acre plot will be granted on one of three terms: Serf-hold, Yeoman-hold, or Sergeantry. A serf is required to pay 50 gp per year in produce or labor to the holder of the manor (master’s choice). He may not move, marry or change occupation without the master’s permission and his heirs must give the master his best animal in order to inherit the property. The serf, on the other hand may not be legally evicted from the holding. A Yeoman holder must pay the master 50gp per year in produce or cash. After the harvest each year the yeoman or the master can terminate the lease. A yeoman can be required to serve in a local militia, which cannot be forced to move beyond the border of the local county or barony. A yeoman is otherwise a free man. A plot held as a sergeantry is similar to a knight’s fee in that the holder (or his son) is compelled to serve for 60 days a year as a foot soldier, archer in the army of a noble. The soldier, however, pays no rent or labor payment for his plot. A greater sergeantry requires the sergeant to serve as a light cavalryman, but due to the expense of maintaining the horse, this sergeant must be granted at least 3 25-acre plots.
Farming Economic Breakdown:
1) The Family and Farm: A farm family with a 25-acre plot produces 150 gp a year in food stuffs. The family consists (on average) of about 3 adults and 2 children.
2) Feeding the Family: a poor meal costs 1sp per person per day, but a farm family can get it for ½ that price since they make it themselves and there is no transport or middleman cost Children eat about ½ as much as the adults. The adults costs 5cp each per day to feed, and the children cost 5 cp per pair per day to feed, so to feed the family for a year costs 73gp-ish (365 days a year).
3) Must pay the Rent: The family then spends 50 gp in rent to the master of the manor.
4) Net Income: This leaves 27 gp per year as “disposable income” for tools, clothes, ale and the like. Many families will supplement this income in various ways, but that is the baseline. Of course, if things go badly in the harvest, the landlord doesn’t often offer rebates. Some families control 2 or more plots, which can leave them quite prosperous, other families only control half a plot, which leaves them wretchedly destitute. You can see that a sergeant’s family has three times the disposable income of other families, but the sergeant must provide his own arms and armor and risks death in his master’s service. The farming is harder on a sergeant’s family too, since one member is gone for 60 days a year.