Taxing the Life out of the Peasantry

Disregarding domain morale, how high of taxes can you levy sustainably? I have a player wanting to play a dark lord and I can already see the orc hordes cheaply suppressing/oppressing the peasantry in the near future. 

IANA, but I think it depends on land quality.  Raising taxes effectively lowers the land quality from the peasants' pont of view.  Land qualities range from 3gp/family land up to 9 gp/family land.

Once you've dropped them below 3-4 gp effective land quality you've gone from taxation into pillage.

I thought the 3-4 gp figure bore a bit more explanation:  It's been many months since I ran the numbers, but if I recall correctly, a peasant family can't actually make ends meet on 3 gp land, with standard taxes and upkeep expenses, so if the land value is 3gp the ruler more or less has to cut taxes.  On the plus side, that gives a bit of a morale bump to the dirt farmers.

So if it were me, I'd set the sustainability floor at 4gp and your dark lord's margin for excess taxation would be the land value minus 4gp.

According to Axioms III, "The Economics of Peasant Families", the average peasant family produces 16.25gp per month and consumes 4.25gp per month. You may also want to take a look at the rules for slave latifundia that I posted on the ACK houserule section of this forum :)