Taxation and War

Domains
A square mile of good farmland typically supports 20 productive households, generating 140 sp/month in taxes. In the River Lands most service is feudal rather than paid, and most of Kallent, Redfort, Herdley etc is enfeoffed to landed knights, with a typical Riverlands manor being around 2-4 square miles of farmland (1280-2560 acres, at 640 acres per square mile).

Each peasant manor has typically 20-80 peasant families, each family generating an average 7sp per family per month. It takes minimum manor revenues from 20 peasant families - ca 140sp/7gp a month over the year - to field one mounted knight (average Ftr 1-3) and a retinue of four men (Ftr 0) in time of war, although at this minimum level such 'poor knights' typically wear only banded or splinted armour, rather than plate. By comparison, to hire furnished mercenary heavy cavalry of equivalent quality can cost around 72gp/month each even in peacetime! It's possible to raise a peasant levy in addition to the knight and his lance, but this is rarely considered advisable except in direct defence of the manor itself.

Yeoman freeholdings typically exist alongside feudal manorial holdings, in variable proportion. Herdley and Talworth have little of a Yeoman freeholder class, whereas in Yggs and Easmoor they are common, with the Royal Domain, Redfort, and Kallent in-between. Yeomen have the same feudal service obligations as do knights. In theory a freehold manor of 20 yeoman families can typically muster ten men 'in prime condition and suitable for men-at-arms status' (labouring male, Ftr 0, hp 1d6+1), with another ten men and women 'capable of wielding a weapon' (active male & labouring female, Ftr 0, hp 1d4+1). Yeomen are traditionally expected to provide their own weapons (usually agricultural implements), train, and feed and supply themselves on campaign for up to forty days; but a wise lord knows not to take this for granted - in practice Yeomen are most effective when mustered in defence of their own homes, or when paid, trained, and equipped by the Lord as Volunteers. Wealthy Yeomen have been known to pay cash in lieu of feudal service.

Typical Servant & Soldier Costs, monthly cost subsumes food and basic clothing (for the unskilled this is the entirety of the expense!), and assumes accommodation is provided. High Skill & above recruits are usually very limited in number, and will cost much more if hired on a short term basis, around x10 is typical.
Unskilled: 1 sp/day, 1gp/month: Lackey, Maid, Labourer, Barbarian, Irregular, or Levy men-at-arms.
Moderate Skill: 2 sp/day, 2gp/month: junior artisan, Green Regular men-at-arms (+1gp/month heavy infantry arms/armour)
High Skill: 3sp/day, 3gp/month: experienced artisan, Experienced Regular men-at-arms (+1gp/month infantry arms/armour)
Very High Skill: 5sp/day, 5gp/month: veteran artisan, Veteran (Ftr-1+) Regular men-at-arms (+1 gp/month infantry arms/armour)
Elite Skill: 9sp/day, 9gp/month: elite arisan, men-at-arms Serjeant (Ftr-1+) (+1 gp/month infantry arms/armour)
Master Skill:  20sp/day, 20gp/month: master artisan, Serjeant Major (Ftr-2+)

Mercenary Companies
Mercenary Companies are supplied according to Contract, normally for a Campaign Season of 3 months at a time. Troops are supplied armed and armoured under the command of a Mercenary Captain. The most commonly employed mercenaries are Heavy Cavalry, such as the Riders of Rohiran used by Easmoor. These are often landless knights (Ftr-1+), at a typical cost of 72gp/man/month, raised in squadrons of 30-50 men. Other somewhat cheaper mercenary types include Tilean Crossbowmen, who employ the famous  Tilean Arbalest (range as heavy x-bow, but dmg is 2d4+2/2d4+2, FR 1/2) with stock and pavis, the greatsword-wielding Cimric Highlanders, Siluras Longbowmen, the Urt River Axemen, Altan Pikemen, Easterling Horse Archers, etc.

Economy & Taxation
In a Martial Domain of 20-50 miles radius such as Kallent, typically just under 1/6 of GDP is taken in taxes by the Lord; of which typically roughly 1/4 is in coin taxes & tolls (usually silver), 1/4 in goods (eg flour tax at the Lord's mill), and 1/2 in direct labour (eg the yeoman's typical 40 days annual service requirement per active person, or the peasant's obligation to till the lord's fields). This amounts to 7 sp per 5-person household or family per month in an average* domain.
Each family has on average two economically active persons, making up are 40% of total population, with the remainder being small children, the elderly, invalid, etc. Typical household GDP is 45 sp/month. Each economically active person produces on average 22.5 sp/month GDP (half of 45 sp/month per household), or 270 sp/year, equal to 13.5 gp per year per person, or 27 gp/540 sp per year per household of 5 (2 active, 3 inactive).

Arcane domains (10-20 miles radius) are less able to tax efficiently, raising 5 sp/family/month from a household GDP of 45 sp/m, while Religious domains (15-30 miles radius) are the most efficient, and can raise 9 sp/family/month, approximately 1/5 of a household GDP of 45 sp/month.
It takes around 4/5 of a typical household's labour to feed itself; attempts to tax beyond this usually result in starvation, declining revenue, and sometimes rebellion.

*The Malthusian Trap sets a typical domain productive population limit of 50-100 people (10-20 households) per square mile of farmland, depending on soil fertility and other conditions. Above this population level overpopulation occurs, no additional revenue is gained, and famine and plague are likely, although actual rural populations have been known to reach as high as 180 per square mile. Below this optimum level fields lie fallow, and revenue declines. Trade and production centres such as cities and mines are an exception.

Coinage
The Pound Sterling is normally an accounting device, nominally 1 lb of pure silver, worth 10gp or 200sp.
Occasionally some realms issue Platinum coins (pp) worth 5 gp. 100pp weigh 1 lb.
The Gold Piece (gp) is typically a 12-carat coin. 100 gp weigh 1 lb.
The Silver Piece (sp) is typically a 12-carat coin. 100 sp weigh 1 lb.
The Copper Piece (cp) is a large copper penny. 25 cp weigh 1 lb.
Common smaller denomination copper coins used by the lower classes are the half pence (0.5 cp, 50 per lb) and the farthing (0.25 cp, 100 per lb).