Here is my attempt at coming up with an average based on what housing costs are in the 3.5 DMG. These amounts are what a family would earn a year (net income). It is not a representation of overall wealth.
I would use a similar methdology. Look at housing costs and assume renting it or paying mortgage as approximately the same. I'd also look at the food costs, and use that as the minimal baseline for a non-starving poor person.
Thus, a serf might pay no rent or taxes (because they own nothing), but be paid money so they can buy food. That food would be the meagerest, but would set a baseline for a poor, but not starving person.
In modern times, a bank would loan out 2-3 times your annual salary for a mortgage. You could use that as guesstimate by quality of house the person lived in. I'm not sure if real medieval banks would loan out that much for a house, but the concept is workable in terms of guesstimating wealth.
You can also work out that the farther back you go, food costs were a larger percentage of income (less spare money for extra stuff). Additionally, that ratio has always been worse for the poor, compared to the wealthy.
So a cross test of the math is that the % of expense to income should be higer overall and the gap would be larger, the higher the income.
From the
d20srd.org:
a poor meal is 1sp
So a poor man with no rent needs 1sp/day so he gets 1 meal a day.
if he rents a room or shanty, that's 2sp/day.
Grand total 3sp/day, or 90sp/month or 1095sp/year
That is for a poor guy with a daily meal and a roof over his head to call home.
Any less than that, and he's starving or does not have a dedicated place to call home (sleeps where he can).
A man with a family is going to need to make more. A man looking to move up (better meals, home, family) is going to need to make more so he can save up, etc.
Luckily, the SRD also shows us that a trained hireling gets at least 3sp/day. This means the lowest paid trained hireling is POOR.
An untrained hireling at 1sp/day is basically mooching off his master for shelter, because he's only getting enough for 1 meal/day.
A guy who can cast a 0th/level spell per day (as in enough demand for) is cranking out 5gp and would be doing quite well.
A Messenger makes 2cp/mile. He need to go 5 miles to hit meal time. 15 miles/day to hit meal+rent. Unless he can get paid going both ways, he's only getting paid every other day. Average walking speed is 4 miles/hour. In an 8 hour work day he's got 32 miles. That's 6.4sp/day. Not bad for a walking job. Obviously, if he's running (short distances, more jobs?) he'll make even more.
If you want the good lifestyle (based on the Inn cost of 2sp+5sp), you'll need 7sp day per family member. The messenger could rock close enough for himself.
Anybody else, the SRD just doesn't offer enough diversity for job classifications. And there's a broad jump in unit of measure for hirelings paid in silver, to a low-level caster doing 0th level spells at 5gp a hit.*
*actually more than that as its times caster level, and the pay-worthy spells are at least 1st level.
This actually reveals a different disparity. Folks talk about casting the Create Food and Drink spell and be rolling in the bucks. That spell costs more to have it cast than to just buy the actual food at 5sp/person.