In the Conan d20 game it's 50% of your PCs' net worth (including magic items!) per month.
I love that rule, even tho I've never used it. It just cracks me up.
PS
Yeah, it really captures the flavor of the stories.
I would say in general 1 s.p./level per day as a default 'lifestyle' expense. (Make this 1 g.p./level per day if you are using a g.p. monetary standard, like default 3.X or 4e.) The characters might spend their money differently - one character is buying booze and prostitutes and another gives it away to churches, their elderly mother, or beggars - but they all are in one way or the other living more grandly as they increase in stature.
If this comes up frequently in a campaign with alot of downtime between 'adentures', you could pay more attention to it as 'FantasyCraft' does.
As far as 'XP during down time' goes, my rule has always been that you can earn 1 XP per day practicing your trade or craft during downtime. So the fighter that wants to go out hunting to keep his combat skills sharp he earns 1 XP per day doing so. If you want to earn XP at a faster rate than that, it involves significant risk and can't be handwaved. Although, I might be convinced to double down time XP to 2 XP per day if the player doubled down time cost of living (representing gym fees, membership in a weapons guild, training with a local gladiator coach, paying for classes with a fencing master, ect.) depending on whether such oppurtunities were available.
Of course, a PC in need of cash can almost certainly make more money practicing their trade than they spend in expenses - especially below level 5 when their lifestyle isn't that much grander than ordinary middle class.
Also, the 'lifestyle' expence doesn't include cost of maintenance to property, taxes on income, wages for hirelings, serious magical research, item creation costs, etc.