All RPGs have ways of separating players from their hard-looted (or maybe even earned) money. Of course, there are equipment expenses, which make up the majority of expenses. Then, sometimes, players need to hire specialists (sages, transportation) or bribe someone in a position of power.
But everyone needs to eat and sleep. In most games I've been in, the GM will charge for a room and meal at the inn, but since the characters rarely spend more than a few days in any one town, and nobody ever seems to buy more clothing :), the idea of a monthly cost of living usually doesn't come into play.
However, many games have rules for such expenses.
In Traveller, it's your Social Standing stat times 100Cr per month, and if you don't spend it routinely, your Social Standing suffers. Players with ships generally also have a ship payment and maintenance costs, but that is offset by their ability to earn money with their ship in many cases.
AD&D (1st ed) had a cost of 100gp per level per month, to be charged automatically by the DM. The stated goal of this rule is to make sure players burn through money, just like the Traveller rule.
Pathfinder has expenses too, but they're not by level, they're by how "high" you want to live. In practice, it means 100gp per month to "live large" - much less expensive than OD&D. Perhaps this rule has changed because of the split between gp and xp, and expected character wealth? I'm not sure, but it certainly is a shift in the way the game works.
In D&D 4e, I couldn't find any equivalent rules, but I'll admit I'm not as familiar with the rules layout and may simply have missed it. I guess it fits the more adventure-centric focus of 4e, though it's possible they just moved those rules to a "Dungeon Mastery" expansion. <shrug>
Games like Mekton, BESM and other light systems don't concern themselves too much with bean counting, preferring to abstract those ideas into the character's background.
So, any thoughts? Traveller and OD&D charge you a lot to cover the various expenses of your Social Standing or Level's equipment and entertainment requirements. Is it even reasonable to port that sort of thing into Pathfinder, or will it skew the "wealth by level" mechanism too much?
I found my D&D 3.5 books, and found that their is a variant rule for cost of living (instead of bean-counting), but it's the same rule Pathfinder adopted - a fixed price for how large you want to live, with 100gp being "pretty large" and 200gp being "like a king". This is still well short of AD&D's "100gp per level" though.