Lets say that a party has recently decided to retire from adventuring to focus their efforts in caring for a city-state in need of benefactors. I want to know what the lowest level a party can be and still safely make enough money to provide the required financial support to care for a city-state's population, without their risking life or limb adventuring.
This isn't an easy question since the economy of 3.5 is so screwed up that it's difficult to determine how much a gold piece is worth, much less how many it takes to equal a nations GDP. So Let's be a bit more exact on what I mean. Let's say the party is in charge of a City-State the size of Rome, with a population of 35,000. They want to generously provide for it's citizens by ensuring that each and every person can live a wealthy lifestyle, which costs 50gp/week/person. That means they need to provide 1,750,000 gp/week.
The adventuring party consists of 5 members, all at or below the party level you chose for your answer, of whatever classes you deem appropriate. The party can work any 'safe' job necessary to help earn the income required. At the time of their retirement they have at their disposal an amount of money expected for a party of their level, based off of wealth/level guidelines, to spend on purchasing items or equipment which would assist in providing for the City State.
If necessary the party can take up to a month's worth of time, starting at the moment of retirement, to prepare for providing for the city. This could be spent building equipment, training underlings, or saving up money for a large purchase; whatever will help them to best provide for their city.
The lucky members of the city are being cared for without being required to earn the support, meaning they can not be utilized as part of the parties money making scheme. However, the party can employ any underlings or hirelings they would otherwise have access to.
Any solution must be sustainable long term, at least until the original party grows too old to continue providing for the city. Bonus points for minimizing cheese factor (though I'm open to answers with some low degree cheese) or for not requiring every member of the party to be equal to the total party level