Well the first thing is what do people want the model to do? And be specific, if you want inflation or deflation or employment or unemployment specify it.
In the meantime here is a relatively simple model one could use to determine the break down of GDP and GDP if necessary.
The Keynesian Model is probably the easiest for the layman, Y=C+I+G+(X-M)
Where Y=Aggregate Expenditure
C=Consumption
I=Investment
G=Government
X=Exports
I=Imports
Punch those values in, and you have a GDP value, since GDP=Aggregate Expenditure=Aggregate Income=Chain Volume. That will give you a snapshot of the economy frozen in time, useful for updates and statistics.
I’ll change the model into a closed system (no trade) to make it easier to understand.
Y=C+I+G
So Y=Aggregate Expenditure (which is equal to GDP)
C=Consumption Expenditure by households
G=Government spending on goods and services
The difficulty is figuring out precisely what goes where, how does one know the ratio of investment to consumption in a given economy, what are its effects and how might the ratio’s shift?
You just make up a formula, since Y is typically known, and G is fairly easy to figure out (even if one only looks at the historical taxation rates of the period).
I would have a formula typically like this for any period before 1750
Y=C4/6+I1/6+G1/6
So consumption by the population is around 4/6th of the economy, investment in new factors of production or technology makes up around 1/6th, while government expenditure makes up around 1/6th (a deliberate over-estimation by around a factor of maybe double, NESer’s need something to play around with).
Consmuption alone does not lead to growth, investment however does, one could then come up with a means of calculating growth according to this method with a formula like
I=% Change in Y-(Markets x Property Rights x Monetary Exchange x Legal Systems x Transportation x NESer Mistakes)
So I alone would equal around 16% assuming it was 1/6th of Y, much to high, to move it down to NESing levels (people do not want a growth rate significantly below 1 like it was before the industrial revolution in aggregate) I would deduct points for each missing factor of growth, all listed in the above formula and assign them all a weighting.
I=% Change in Y – (M1/9,PR1/9,Mo1/9,LS1/9,T1/9,NESM4/9)
So now you could using your judgement calculate the growth of the economy. Any government expenditure that is invested in the economy could also be rolled into I with regards to growth, perhaps at a reduced rate call it 2/3rds of the initial value or some such.
International trade, that’s the difficult bit, I’m yet to come up with a simple means of managing trade. For the moment just filling in arbitrary export and import values might work, with import and exports being = to 0 if the one is taken from the other for the system as a whole.
Much as I want to answer your question FC (Das this might help as well), its to general to answer properly, I can tell you that typically in pre-industrial economies (at least before the United Provinces came into being and before the Glorious Revolution in England at least with regards to Europe), states had no sure form of income. Some had reasonably stable ones, as Das has noted, tolls, and tribute are really only the stable sources of income open to a pre-industrial state. Typically all a state did in wartime was get it’s predatory on, some states had “usual” means of sourcing funds, a favourite was a special tax on certain goods, other favourites included increasing inheritance taxes (another reasonably stable source of income in normal times), forced loans and a whole other gambit of means. What is essential to remember is this tended to be extraordinary taxes, in that they were dismantled after the extraordinary event which created them ended. Some states have relatively speaking have a more advanced system of taxation, since the natural proclivity of states is just to take from the areas under threat, a special tax on coastal shipping to stop pirates etc, the more efficient states tended to be able to spread this risk out between a much large share of taxpayers spreading the economic damage of a confiscatory tax.
So in short states did not have stable finances, on a year to year basis they might fluctuate hugely, since the majority of the states income came from agricultural sources which vary from year to year significantly. Having these sources paid in cash helps a great deal, since when one is the state one does not need to have a sliding scale, you just specify an amount, and regardless of the leanness of the system or the bounty everyone pays that amount (some flexibility is typically built in, if only note to bankrupt your whole peasant underclass when they get a few bad seasons in a row).
Here’s a general rule, most something like 90% of all state expenditure throughout history has been for military purposes. In terms of total economic output, the state seldom took more than 5% of GDP creeping up to 10% in the most militarised states. Increases could not be much higher in a great many cases, since people could not pay for them in cash or in transportable goods… taking peoples homes and other fixed assets is a sure way to lose any support.