Economy Simplified

This version of the economy system is "5th-ified" and I can't be convinced to expand it further.
The economy system of 5th Edition is said to be lacking and alas I agree but I've looked at Grain Into Gold and I'm not interested in running an economy simulator. The price list from the Players Handbook stands, we'll just make the average man's wage fit. Note however that Hârn is more poorly and though their detailed list of wage converts well with this system.
On page 159 of the Players Handbook we learn that unskilled workers are paid 2 sp per day, while skilled workers (anyone performing a service requiring proficiency) are paid 2 gp per day. On page 158 we learn that unskilled workers tend to live a poor lifestyle, while the skilled ones lead a comfortable life. The price of each lifestyle neatly match the wage of the worker. We assume the same is true for other social statuses, and can therefore construct our first table. I've converted the daily income and lifestyle to monthly.

Social status	Minimum income/month	Lifestyle/month	Profit/month
Poor (unskilled labourer, costermongers, peddlers, thieves, mercenaries)	60 sp	60 sp	0 sp
Modest (soldiers with families, labourers, students, priests, hedge wizards)	300 sp	300 sp	0 sp
Comfortable (merchants, skilled tradespeople, military officers)	600 sp	600 sp	0 sp
Wealthy (highly successful merchants, a favoured servant of the royalty, or the owner of a few small businesses)	1200 sp	1200 sp	0 sp
Aristocratic (politicians, guild leaders, high priests)	3000 sp	3000 sp	0 sp

As you can see, this creates ranges for each social class. The minimum wage of a Modest NPC is 300 sp/month, but she can earn up to 599 sp and still be considered modest. The minimum wage is also the average wage for that social class, as we learned from PHB 159. This is the closest we'll get without messing with anything, and if we wanted we could stop here. We could say that the average NPC of any given social class makes just enough to support their lifestyle, but not enough to save anything. A few would be able to save or spend extra money, and a few wouldn't be able to keep up. This is actually all you need, but it isn't entirely satisfactory.
First of all, the lifestyle cost is intended for an adventurer. It assumes they buy all their meals, rent lodging and pay for the reparation of weapons and tools. Clearly, this is not the case for all NPCs. Thus, we have to reduce the lifestyle expense of NPCs to accommodate for preparing their own meals, fixing their own stuff and owning their own home. Disclaimer: Many NPCs, especially of the lower classes, would probably in fact be renting and not owning, while the higher classes would not be preparing their own meals or fixing their own stuff. Regardless, I have decided to reduce all lifestyles equally for the sake of 5E simplicity. As we will see later, it generally works out anyway. The question is how much we reduce it by. Of course, there are not guidelines in the books regarding this, but the average today is between 40 % and 70 %, from what I can gather. I'll go with 50 %.
Second, the table only accounts for the NPCs lifestyle expenses. They also have a business to maintain. This is where the difference from our reduction in lifestyle evens out. The poor classes might have to rent, but they generally don't have to maintain their business, as they work for someone else. The homeowners, however, often have a business maintain. So now we pull up the DMG on page 127 and look at maintenance costs. Sadly, it is a complete mess. The maintenance for a farm is 5 sp per day, and that includes the pay for a skilled worked and two unskilled ones. The DMG states we can find the details of skilled and unskilled workers in the PHB. We already did that, and a skilled worker earns 2 gp per day. The minimum maintenance cost of a farm would therefore be 2 gp (for the skilled worker) plus 4 sp (for the unskilled workers), and that is excluding repairs, feeding the animals, maintaining equipment etc. What WoTC were thinking when they made those tables we will never know. I tried playing around with the numbers for maintenance cost for a while, and ended up with it being a third of monthly income. Remember, the farmer farms his own farm, so again the cost is lower than for an adventurer.
Now the table looks like this:

Social status	Minimum income/month	Lifestyle/month	Upkeep/month	Profit/month
Poor (unskilled labourer, costermongers, peddlers, thieves, mercenaries)	60 sp	30 sp	20 sp	10 sp
Modest (soldiers with families, labourers, students, priests, hedge wizards)	300 sp	150 sp	100 sp	50 sp
Comfortable (merchants, skilled tradespeople, military officers)	600 sp	300 sp	200 sp	100 sp
Wealthy (highly successful merchants, a favoured servant of the royalty, or the owner of a few small businesses)	1200 sp	600 sp	400 sp	200
Aristocratic (politicians, guild leaders, high priests)	3000 sp	1500 sp	1000 sp	500 sp

It is starting to look like something. We probably want to add some taxes though, these peasants seem a bit rich. There are as many ways to tax poor peasants as there are rogues in a Roll20 campaign, but yet again I went for simplicity: A form of poll tax. Every commoner over the age of 14 pays a set tax based on his social status, not his actual income. I am a kind ruler, the tax is 10 %. You can obviously set it as high or low as you want according to the avarice of your ruler or whether or not money is needed for an incoming war/building of a giant statue. The commoners will start losing money at 17 %.
With taxes, our final table looks like this:

Social status	Minimum income/month	Lifestyle/month	Upkeep/month	Taxes/month	Profit/month
Poor (unskilled labourer, costermongers, peddlers, thieves, mercenaries)	6 gp	3 gp	2 gp	6 sp	4 sp
Modest (soldiers with families, labourers, students, priests, hedge wizards)	30 gp	15 gp	10 gp	3 gp	2 gp
Comfortable (merchants, skilled tradespeople, military officers)	60 gp	30 gp	20 gp	6 gp	4 gp
Wealthy (highly successful merchants, a favoured servant of the royalty, or the owner of a few small businesses)	120 gp	60 gp	40 gp	12 gp	8 gp
Aristocratic (politicians, guild leaders, high priests)	300 gp	150 gp	100 gp	30 gp	20 gp

This seems okay. We see that the commoners of DnD are not so freakishly poor as some believe, and if a community pools together its savings they might be able to pay some helpful heroes. Quite a large sum flow through them in the course of a week, but they don't have a lot to spend. Many of them have a family to take care of, spends some nights a month drinking at the tavern, or have to pay for unexpected stuff not covered by lifestyle and business maintenance, so their actual savings each month will generally be lower than their profit. A poor worker can spend 1 silver a week on beer at the tavern, but he won't be saving much if that's his choice. It will take a modest NPC a bit over a year to save up for a healing potion, two years for the antitoxin for his sick daughter. The comfortable smith can buy a second set of tools after 5 months of hard work, while the successful merchant can buy a magnifying glass to more accurately assess gems in about a year. After little more than a month the minor noble can buy a vial of acid to stop the mouth of that pesky guild leader. Everything seems to line up quite nicely with the prices in the PHB.
However:

• Money is an abstraction. More often than not, the NPCs will not see their pay in the form of coins, but of produce or services. The same applies to their taxes.
• The monthly costs are also abstractions. Some NPCs might get their pay on a weekly basis, while others only get their money a few times a year.