In Underworld & Wilderness Adventures, there's an often-ignored rule: "Player/Characters must pay Gold Pieces equal to 1 % of their experience points for support and upkeep, until such time as they build a stronghold." (p. 24.) Delving Deeper has a similar rule that specifies the time period as per month.
In a comment on yesterday's post about taxes, Rod Thompson said that 1% is about what the base tax rate was in Roman and medieval times. So it seems kind of accurate... except the charge is for upkeep, not just a tax. Now, it's true that, over the long term, a 1% tax on experience points, rather than income, will result in a PC paying much more than 1% of income, since it's a tax on cumulative income, rather than income in a given time period. But should the upkeep cost include taxes, exclude taxes, or be replaced as a tax?