I once read this story about a tailor who had a client who was in a great hurry. The tailor measured the man's thumb circumference, and told the client he could go.
The client protested - surely that wasn't enough. The tailor answered that if he had the man's thumb circumference, he could multiply to get the wrist circumference. If he had the wrist, then he could deduce the neck, and from the neck the waist etc etc.
What does this have to do with a fantasy economy? Well everything is interconnected. The price of a simple sword is based on the cost of iron, but also on the labor cost for the smith who made it, and perhaps also of the merchant who brought it to you. The smith's labor cost are in part based on the cost of food and fuel, and those also have labor costs. Meanwhile the merchant has to do run a profit and he has staff to pay, taxes and bribes to deal with and all sorts of complication.
Because of this, and because our historical records of prices in the middle ages are not great (and vary a lot depending on the era and the location), it's *really hard* to have a price list in gaming that doesn't collapse upon close scrutiny. Usually related items make sense - if a short sword is 10 gp, the long sword might be 15 gp for example. But is the price of that sword reasonable when compared to an ox? A cart? A lantern? A meal? Maaaaybe?
I'm not suggesting to redo entire price lists so they are all "correct". To do so would be a herculean task and not the best use of your time as a GM. But sometimes you *do* want to verify if a price is "reasonable". And for that, you need a yardstick. You need a value that you have decide "of all the prices in the book, *this* one is correct". This yardstick should be something pretty fundamental.
I propose the cost of living - specifically, the cost to hire a general laborer. The laborer is a poor person - but they won't work for an amount that won't somewhat meet basic living needs. So this value is tied to a lot of things.
In older editions of D&D, this was 1 sp/day. In 5e, the "poor" costs of living is 2 sp/day, or, in Yoon-Suin terms, 1 rupee a day (1 sp/day is considered "miserable"). This is a doubling, but it's still in the same ballpark. We aren't after an *exact* value here, but rather a nice round number that is plausible and we can use that number to figure out other things.
Is this value plausible? Well in ancient Greece, half a drachma (a 4.3 gram silver coin) could provide for a poor citizen so we are certainly in the right ballpark.
Warhammer also has charts of yearly income for various types of hireling. A skilled mercenary (ie, a PC...) can expect a yearly rate of 25-50 gp a year, ie roughly 0.5-1 gp a week (in this system 1 gp = 20 sp) or a few sp a day. Meanwhile, a peasant will make about 12-15 gp a year. Clearly in this system 1 sp is worth more... but we are still in the right ballpark.
So this is what I propose - Whatever the game system you are using, look at the cost of living as your fundamental economic unit, whenever you need to figure out if the value of something is plausible. Both D&D and warhammer frpg have laid those out pretty clearly.
I'll be writing more about fantasy economies, but this is the foundation upon everything else will be built.