The Fantasy Trip (In the Labyrinth) had some excellent rules for this and, I think, they can be ported to D&D without much trouble. Here's an abreviated form from the now missing World of Etan documents:
LIFE BETWEEN ADVENTURES
Risk Rolls:
Everyday life isn’t as exciting and rewarding as going on adventures, but it isn’t
completely safe and dull either. Whenever a character takes more than a week off from
adventuring make a “risk roll” on three dice for each week that passes.
For characters living a quiet life, the risk numbers are 3/18 – a roll of ‘3’ is a “low”
roll and an ‘18’ is a “high” roll. For moderately active characters, the risk numbers are
4/17 – 4 or less is “low” and 17 or higher is “high.” For very active characters, the risk
numbers are 5/16 – 5 or less is “low” and 16 or higher is “high.” Beyond this point, the
characters are actually adventuring and should be played as such.
On a “low” roll the character gets 125 experience points and a financial windfall
equal to one week’s normal upkeep costs (or one week’s income, whichever is more).
On a “high” roll the character suffers a financial loss equal to one week’s normal
upkeep (or one week’s income, whichever is more) and must make a three die roll
against his highest attribute to avoid taking damage from illness or injury.
Failure means that the character takes damage equal to half his Strength. Critical failure (a
17 or 18 on this second roll) means that the character takes damage equal to his full
Strength, reducing him to St 0. As usual when a character is reduced to St 0, it is up to
the player as to whether or not the character dies from this.
Note that the die roll against the highest attribute is made against the base
attribute, unmodified by armor, magic items and the like. The only things that affect
this roll are Curses, Wishes, and other things that modify die rolls directly rather than
modifying attributes.
Upkeep Costs:
The minimum upkeep cost for a subsistance-level existence is $20 per week.
Spending less than this has bad effects: Spending less than $20/week adds +1 to your
weekly risk roll (making bad rolls more likely and good ones less so.) Spending less
than $10 per week adds +2 to the risk rolls.
If you spend $40 per week or more on upkeep, you pass into the realm of the
“middle class” – master craftsmen, professionals of various sorts, and especially well-off
peasants.
If you spend $80 per week or more on upkeep, you pass into the realm of the minor
nobility and similar well-off sorts. At this point, you should probably hire a servant and
start worrying about style and fashion.
If you spend $160 per week or more on upkeep, you pass into the realm of the major
nobility. At this point you should definitely hire at least one servant and can start
thinking about actually setting style and fashion.
Spending $320 per week or more on upkeep puts you into the lower end of “filthy
rich” territory. Before, you may have had one servant and used the best
accommodations available. Now you’ll have multiple servants and use accommodations
that were prepared especially for you.
Jobs:
Characters can work at various jobs between adventures (or instead of adventuring)
in order to earn money and/or kill time. The different jobs described below are
intended as a useful sample; more exist. The description of each job includes:
1. Pay: How much the job pays each week when one first takes it. Obviously some
jobs pay more than others. Most jobs, however, pay better the longer one stays with an
employer (or works at it, if one is self-employed). Typical raises are 2% per year: A 35
year veteran will typically earn twice as much as a character starting out.
For some jobs (particularly “free-lance” ones) pay is rolled randomly and will vary
from week to week.
2. Availability: How hard the job is to find. This is given here as the base number of
dice rolled, using the general rules for searches. The base time to find a job is one day.
Some jobs are “free-lance”; one can start working at them without a die roll.
However, such jobs usually have variable pay: Sometimes you make a lot, other times
you might not make anything.
3. The Minimum Risk: Characters can choose to live more dangerously than their
jobs require, but cannot choose to live less dangerously. Most jobs are quiet and
peaceful (minimum risk 3/18), but some are inherently...interesting.
4. The Requirements: The minimum by way of skills and attributes needed to get
and keep the job.
Sample Jobs:
Lower Class Jobs
UNSKILLED LABOR: Pay $20/wk; Availability 1 die; Risk 3/18; No requirements.
PERSONAL SERVANT: Pay $5/wk plus upkeep; Availability 6 dice; Risk 3/18; No
requirements.
CLERK: Pay $25/wk; Availability 3 dice; Risk 3/18; Requires Literacy.
MUNDANE WORK: Pay 20-120+/wk; Availability 3 dice; Risk 3/18; Requires a
Mundane Talent. Pay is $20/wk per IQ point worth of mundane talent, doubled if you
have your own shop.
PEASANT FARMER: Pay 20-120+/wk; Availability special; Risk 3/18; Requires
Farming, land to farm. Availability is automatic if you can get land to farm. Pay
depends on how much land you have to farm and on whether and how badly the local
nobles oppress the peasantry.
SAILOR: Pay $10/wk plus upkeep; Availability 3 dice; Risk 4/17; Requires
Seamanship.
Professional Jobs
ARMOURER/SMITH: Pay $50/wk; Availability 4 dice; Risk 3/18; Requires
Armourer or Goldsmith. Pay is $100 if you have your own shop.
MASTER ARMOURER: Pay $75/wk; Availability 4 dice; Risk 3/18; Requires Master
Armourer. Pay is $150 if you have your own shop.
PHYSICKER: Pay $60-120/wk; Availability 6 dice; Risk 3/18; Requires Physicker
and a Physicker’s Kit. Pay depends on the size and quality of your ‘practice.’
MASTER PHYSICKER: Pay $100-200/wk; Availability 6 dice; Risk 3/18; Requires
Master Physicker and a Physicker’s Kit. Pay depends on the size and quality of your
‘practice.’
SCHOLAR: Pay $85+/wk; Availability 4 dice; Risk 3/18; Requires Scholar. Add $15
to weekly pay for each language known (full knowledge only) beyond your first.
PRIEST: Pay $40/wk; Availability 4 dice; Risk 3/18; Requires Priest
JOURNEYMAN CHEMIST: Pay $60/wk; Availability 4 dice; Risk 5/16; Requires
Chemist
MASTER CHEMIST: Pay $110/wk; Availability 4 dice; Risk 5/16; Requires Chemist
and Dx 12+. Note that taking the Chemist talent a second time adds +3 to Dx for this
job. Note also that a Chemist can (sometimes) make even more money by making
potions and selling them for whatever he can get.
JOURNEYMAN ALCHEMIST: Pay $65/wk; Availability 4 dice; Risk 5/16; Requires
Alchemist.
MASTER ALCHEMIST: Pay $130/wk; Availability 4 dice; Risk 5/16; Requires
Alchemist, Dx 12+. Note that like Chemists, Alchemists add +3 to Dx for this job if they
take the talent a second time. Also like Chemists, Alchemists can (sometimes) make
even more money by making potions and selling them for whatever they can get.
BUILDER: Pay $70/wk; Availability 4 dice; Risk 3/18; Requires either
Architect/Builder or Shipbuilder.
ANIMAL TRAINER: Pay $90/wk; Availability 5 dice; Risk 4/17; Requires Animal
Handler and Vet. If training dangerous animals, increase Pay to $165/wk and Risk to
5/16.
ENTERTAINER: Pay (2d-2)/wk times a multiplier; Availability freelance; Risk 4/17;
Requires talents or spells useful for entertainment. The pay multiplier is $5 for each of
the following talents: Bard, Charisma, Sex Appeal, and Master Bard, plus $2 for each
showy spell which, in the GM’s opinion, could be used to entertain an audience. A Bard
with Charisma would earn (2d-2) x $10 per week at this job, while a wizardly
entertainer with Charisma and five showy spells would earn (2d-2) x $15 per week.
Combat Jobs
GUARD/GARRISON DUTY: Pay $30-300+/wk; Availability 3 dice; Risk 5/16;
Requires loyalty, discipline, and fighting ability. Pay depends on how much of each of
the three requirements one has. A punk with minimum of each will get the minimum
pay, a typical “city guard” with moderate amount of each will get $60/wk or so, and an
elite mercenary with legendary amounts of each will get the maximum.
COMBAT DUTY: This falls under the category of being hired for an actual
adventure and should be treated as such.
Illegal Jobs:
In addition to the usual effects, each ‘bad’ risk roll made will add to the character’s
chances of being recognized as a crook and gaining the local “law” as an enemy.
THIEF: Pay (2d-2)/wk times a multiplier; Availability freelance; Risk 5/16; Requires
talents and spells useful for stealing stuff. The pay multiplier is $10 for each of the
following talents: Thief, Silent Movement, Remove Traps Master Thief, and Spying,
plus $5 for each spell which, in the GM’s opinion, would be useful for sneaking around
and stealing stuff.
BANDIT: Pay (2d-2) x $30/wk; Availability freelance; Risk 5/16; Requires three
weapon skills.
CON MAN: Pay (2d-2) x IQ x $1 to $5/wk; Availability freelance; Risk 5/16;
Requires Charisma; Sex Appeal, Bard, Alertness, and Business Sense also help. The
pay depends on how many of these talents the character has; the multiplier is $1 per
talent.
Wizardly Jobs
These all require that the character be a wizard.
APPRENTICE/ASSISTANT WIZARD: Pay $25/week; Availability 3 dice; Risk 3/18;
Requires the Aid spell.
VILLAGE/HEDGE WIZARD: Pay is IQ squared divided by two (e.g. $50/wk for IQ
10); Availability 4 dice; Risk 3/18; Requires a wizard’s chest or a tabletop lab.
TOWN WIZARD: Pay is IQ squared rounded to the nearest multiple of $5 (e.g. $120
for IQ 11); Availability 5 dice base less one die for each item creation spell known
(other than Staff or Staff of Power); Requires a full wizard’s lab.
WIZARDLY THIEF: See THIEF under Illegal Jobs, above. A wizardly thief is just a
thief who uses spells instead of (or in addition to) talents.
WIZARDLY ENTERTAINER: See ENTERTAINER under Professional Jobs, above.
A wizardly entertainer is just an entertainer who uses spells instead of (or in addition
to) talents.
COMBAT WIZARD: See Combat Jobs, above.
