Here is how I handle upkeep in my DF game:
Pay once for any gap of time between sessions. One week, two weeks, a year, whatever. One payment for the downtime. This avoids becoming broke from real-world gaps in time that translate 1:1 to game time. You are assumed to have worked freelance, gotten a job, lived on the cheap, done some successful gambling, etc. - depending on your style.
You can skip the costs by living on the cheap, normally, but you cannot take advantage of any in-town benefits (learning skills you can't self-teach, learning new spells, getting rumors, etc.)
Costs are calculated in this way:
Basic Upkeep: $150/week
Special Rations: Pay full cost for one week's worth of special food (21 meals worth of special rations)
Any spendy disadvantages: +10% per -5 points unless a higher cost is specified.
Any spendy quirks: +1% per -1 points.
Martial Artist: +100% cost.
So a character with Compulsive Carousing [-5] pays $150 + 10% = $165 as normal upkeep.
A character with Compulsive Carousing [-5], Compulsive Gambling [-10], and Gluttony [-5] would pay $150 + 40% = $210 as normal upkeep.
Martial artists pay $300 base - high, but they're spending a lot of special food prepared in special ways, teas, incense, etc. Yes, they effectively cost more than drunken spendy types, but $300 is basically a ceiling as they rarely have other costly traits. They also don't have to pay for new Chi-based skills or powers they learn, because they just meditate them into being.
Disadvantages like Miserliness do not reduce upkeep costs, but will require a roll to replace equipment that is still functional, or to live above one's means, or spend on extras of any kind in town.
Also, on a related note, here is how we deal with rations:
Big Dungeons & Bacon