Alright. So I have no idea if my calculations are accurate or my assumptions reasonable. Even if they are off a bit though, I think it provides an interesting look at the economic buying power of each class. This can then be used as a jumping off point to determine reasonable prices for services and goods based on how prevalent or accessible they could be for a given economic class (ie cantrips and 1st level spells are probably pretty common and accessible, 2nd level spells are less common, and higher level spells are generally accessible only to the wealthiest or most connected).
Good job! Let's tackle this task, then. I am curious whether we'll collectively be able to piece something good-looking! Your assumption is that 1st level spells should be "pretty accessible" and 3rd to the elite. Let's work from that.
Assumption 1
In the PHB, they provide the lifestyle cost per day on page 157. However, I thought it might be better to use the prices on page 158 detailing the price of food per day.
I'd use the lifestyle cost per day with a discount ; since inn stay, as a temporary accomodation, should cost more than a long-term rent.
Assumption 2
I read somewhere that in 1900 the average percentage of income spent on food was about 40%. Eberron certainly doesn't look like Earth in 1900, but I thought it to be a reasonable price point for this exercise.
This could be an average with huge variations according to social class. In this source (in FR, sorry, that's what I stumbled upon googling)
Les consommations populaires dans la seconde moitié du XIXe siècle à travers les monographies de l'École de Le Play - Persée, where they discuss the recordings of a 19th century mining engineer with an interest in sociology, who detailed popular budget in the 1880s, they find that the average part of the income spent of food to be 40-60%, with half of the surveyed families spending more than 50% on food. The surveyed families went from proletarian (with an annual budget of 500 francs) to upper class (with a budget of more than 10 000 francs).
Assumption 3
As the buying power of a class increases, the percentage of the income spent on food drops. I have no idea if this is true, but it seemed like a reasonable idea to me. I kept it at 40% for Squalid, Poor, and Modest. Then I dropped it to 30% for Comfortable, 20% for Wealthy, and 15% for Aristocratic.
That's probably a good rough estimate, since the same source gives 70% and 15% as extrema for the analyzed budgets, and they don't have "aristocratic" budget in the lot, peaking at wealthy.
Assumption 4
I figured the average family size was 5 (two parents and three children). I assumed food prices would be stable for each member of the household, so the cost per day was multiplied by 5 to account for each person.
Yes, but I guess most people wouldn't be buying their meal at inns. Maybe the price list is for eating out, where there is a markup compared to the raw food? Just a random tought.
Once again, I have no idea if my assumptions are reasonable, but I figured they were as good a place to start as any.
Based on this, I got the following numbers.
Food Cost for Family of 5
Lifestyle | Week | Month | Year |
Squalid | 1 gp, 5 cp | 4 gp, 2 sp | 50 gp, 4 sp |
Poor | 2 gp 1 sp | 8 gp, 4 sp | 10 gp, 8 sp |
Modest | 10 gp, 5 sp | 42 gp | 504 gp |
Comfortable | 17 gp, 5 sp | 70 gp | 840 gp |
Wealthy | 28 gp | 112 gp | 1,344 gp |
Aristocratic | 70 gp | 280 gp | 3,360 gp |
Total Income Estimate Family of 5 (Extrapolated from Food Cost)
Lifestyle | Week | Month | Year |
Squalid | 2 gp, 6 sp, 3 cp | 10 gp, 5 sp | 126 gp |
Poor | 5 gp, 2 sp, 5 cp | 21 gp | 252 gp |
Modest | 26 gp, 2 sp, 5 cp | 105 gp | 1,260 gp |
Comfortable | 58 gp, 3 sp, 3 cp | 233 gp, 3 sp, 3 cp | 2,800 gp |
Wealthy | 140 gp | 560 gp | 6,720 gp |
Aristocratic | 467+ gp | 1,867+ gp | 22,400+ gp |
I think this can provide a "good enough" baseline for economic purchasing power based on class. We can start making some reasonable guesses at price points for different services based on how likely they are to be available to a certain economic class (also considering lower classes may need to save for certain goods and services or seek second/third-hand items). Of course, game balance for PCs also needs to be considered, but only to a point. A DM is free to adjust the monetary rewards and treasure an adventuring party can obtain.
The poor-to-wealthy income ratio you get (26) is quite reasonable compared to the historical survey mentionned in the above source (25). So, it's extremely interesting that the table you calculated is capable of producing a realistic result of the wealth distribution. I'd posit that upper wealthy and aristocratic are even higher given historical Gini's ratios, but that's very interesting nonetheless.
I'd say the 6 person out of 10 as commoner and laborer mentioned by Keith Baker in the 3rd ed Eberron book would make them Poor, not Squalid. Even if you have a very bleak view of the 19th century working class, they would not on average sleep under bridges and sneaking into barn for shelter...
That' would mean our family of 5 would need to make 252 gp a year. On the 5 members, 4 would certainly be working. (In 1833 in the UK, the Textile Factory Act prohibited children under 11 to work more than 9 hours a day and 12 hours under 18). With 5 working members, achieving a poor lifestyle means 50gp 4 sp a year, based on 300 working days (even if there is no Sunday rest to observe, I guess as many holidays exists in Khorvaire) means 1.68 sp a day (which is coherent with the 2 sp/day if you integrate a discount for child labor, who are certainly not earning as much as their parents). Of course, when a family gets more inactive members (either because of old age or because of many younger children not old enough to be allowed to work in a factory or mine), they can certainly drop toward squalid living conditions.
If you increase the food-to-income ratio a bit and consider that eating at home should cost less than eating out, you can even get a coherent result of the 1 sp/day cost for unskilled labor mentionned in the Eberron sourcebook for Sharn hired help.