First, let's make it clear: the flat support and upkeep fee is meant to cover all sorts of little details it wouldn't be fun to track, like how many candles you have to buy every month to make sure your house has lighting. At the very least, it covers a place to sleep and enough food to get by on a day to day basis. A lot of this stuff can be bought on an individual basis, and must be bought that way when provisioning an expedition or when the character has less than 50 xp.
I'd even rule that characters moving to a new location must pay for stuff item by item until they've earned some experience in the new location. In other words, experience is tied to reputation.
What the support costs cover, roughly in order of acquisition as the character's level increases:
- room and board,
- rent on a more permanent home, at later levels,
- provisions for the same,
- repair costs for weapons, armor, and living area,
- social obligations and costs to maintain reputation,
- minor servants.
- 10 gp/month = (semi-)private room
- 100 gp/month = small suite of rooms, or a one-room cottage
- 1,000 gp/month = large home
For small provisions, rather than listing them in advance, if a player asks "do I have a kettle?" figure out the cost of the item, double that, and if it's less than the monthly upkeep fee minus the base cost for the size of the home (listed above,) assume they have it. They same goes for "can I scrounge up 30 candles for a ceremony?" or similar questions: total the cost for the quantity desired, double it, and compare to the monthly upkeep fee -- again, adjusting for the cost of the home. Livestock must be purchased once, but maintenance costs for keeping them alive can be figured as for provisions.
Similarly, if the cost of a weapon or piece of armor is less than half the monthly upkeep fee, assume the character can pay for repairs to that item.
Minor servants -- people who cook, clean, fetch and carry, rather than guards or specialists -- can be assumed to be included in the operating costs of a home. Subtract the base cost of the home from the monthly upkeep fee, divide by 10, and then divide by the monthly cost for a servant (usually 1 gp) to find out how many servants are included in your retinue.
Taxes and social costs (throwing parties, carousing in bars, supporting charities) are the most fluid of the costs. Players are bound to want to cut their costs without taking the obvious recourse of waiting until name level and building a stronghold, and this is the most likely spot where they will try to skimp. Ask the players how they plan to do so, and how much of their costs they are attempting to cut; use these to figure out potential downsides to their plan. Skipping taxes would require rolls to avoid getting caught; skimping on social costs result in reaction penalties, being charged outrageous fees, and similar side effects.
There are other ways to skimp on costs. Living in a smaller home would have an effect on perceived social standing; skimping on miscellaneous upkeep results in weapons that break, running out of food at home, and other problems.
Players can specifically elect to build a much larger home than needed, if they have excess money and can purchase or secure land to build on, in which case you do not subtract the base monthly cost for the home from monthly upkeep, giving a little more room for maintaining costs without penalties. Keep in mind that characters who pay 10 gp/month on upkeep, but live in a large mansion they've picked up cheap, are probably going to have some unusual reactions.
Edit: Weird, I specifically scheduled this for tomorrow, because I figured two posts in one day was too much. I'll probably take it easy and working on LZ tomorrow, then.