The majority of noble income comes from land held in fief - fiefs, whose income is determined almost exclusively by population. While there is a level of individual variance, the 'base income' of a fief is based on the following formulas:
SIZE FORMULA RANK 0-1500 1x Squire-Knight 1500-10,000 0.18x + 1230 Knight-Baronet 10,000-150,000 0.05x + 2500 Baronet-Baron-Viscount 150,000-500,000 0.03x + 5500 Viscount-Count 500,000-1.5M 0.01x + 15000 Count-Marquis
This income is then translated into the fief manager, where approximately 1/3d of that base income is 'fixed' - domestic, non-market driven income - and approximately 2/3ds of that income is 'trade-dependent', based on the relationship of that fief to a market or court buyer. These distinctions and the amounts assigned to each particular income source are mostly cosmetic, but they have an IC reality - so that if a portion of someone's income come from salt mines and two houses are competing in the salt trade, both may see reduced income as a consequence, as is presently the case between House al-Malik and House Decados.
There is additional other income on a fief writeup - usually 1000 firebirds/year for the knight's fee paid down by the lord's feudal superior as well as profits from offworld holdings, other enterprises or stipends received from military or political offices. Guild and Church incomes use their own rubrics for determining profits, though they are also fundamentally based off population - in the case of the guilds, the population of the lands they service and in the case of the Church, the population of their dioceses.
It is important to note that all incomes assume subinfeudination, employees and the like - so that the income of a viscounty assumes there are barons, baronets and knights in the appropriate places, the income of an archdiocese assumes bishops and the income of a guild business assumes managers, chiefs and associates all toiling way. This is the same on the way 'up' - feudal payments up the chain, tithes and the like are not tracked as expenditures or income but are assumed to occur in the background that generates the spendable cash available to an estate holder.
Estate holders also, of course, have expenses; while they are detailed in their own files, approximately 1/3d of a fief's income goes towards 'largesse' towards lesser lords and military forces in the form of gifts, feasts, rewards and the like, approximately 1/3d goes towards the peasantry as 'works and welfare' - the maintenance of public thoroughfares, days off, charity, etc and a substantial portion goes towards maintaining the lord's estate. We track cash expenditures largely only for specific one-time expenses, equipment, extra military units and the like - ordinary extravagance can be assumed in a fashion in keeping with the Living Expenses file.