The PHB & DMG say that PC classes, Monsters, and NPCs use different rules. It says the specific beats the general. It says nothing about different costs for players vs NPCs.
The say nothing about costs or economics of an NPC. So since it so clearly says that classes, monsters and NPCS use different rules, why would you assume they would use the same rules for economics? IMO, that's a pretty big stretch... "Oh, the rules says that PC and NPCs don't use the same rules for experience and spell casting etc, but since it says nothing about economics, I'm going to assume that PC and NPC economics are the same."
It says GMs can change things as they see fit and that no rule can cover every circumstance. It doesn't say "any time the rules conflict with flavor text, assume it only applies to PCs".
What flavor text?
All in all, it seems like an extreme amount of mental gymnastics to avoid saying "that doesn't make sense, the mechanics and flavor text are internally inconsistent."
Seems like a lot less gymnastics than saying the rules apply to NPC economics when the rules say nothing about such.
This is far from the most critical flaw in 5e. If we cannot evaluate it without attempting to rationalize it with unwritten rules, how can we ever discuss anything that is important?
Agreed, because it's not really a flaw in 5E. NPC economics are simple a part of the fantasy world that most people just don't care about, have no interest in worrying about, and the rules don't waste pages trying to justify.
Since you want to try and look at economics of the real world, try equating an adventurer to a traveler. Sure, I can travel around walking by foot, living out of garbage cans and eating poorly for pretty cheap. But if I want to live as a traveler in the same manner as I do as a commoner, it is grossly much more expensive. Instead of buy $10 of rice and beans to eat for a week, I've got to hit up Taco Bell or Filberto's for a cheap bean burrito for $5 per meal (a 10 fold increase in cost). Or instead if I live well, the grocery store a $100 of a week's groceries including lots of protein, sides, etc, I've got to spend $40 for a meal at a restaurant. Instead of a clean cheap apartment in a poor part of town for $350/month, I can spend $50/night ($1500/month) on a cheap motel in a poor part of town. Or for a higher standard, $750/month for a nice apartment or $150/night ($4500/month) for a business class hotel.
It's pretty easy to see that an adventurer's (a person who's not always in the same spot and relies on other service industries) costs of living are easily 5-10 times more than someone who lives and works in one place. So, since they PHB/DMG don't have economics of NPCs, why don't you just assume they are 10-20% of an adventurer's costs and see where that leaves your world building.