In D&D, silver is a lot more expensive than on Earth, because there is a constant demand for powdered silver to create holy water. This consumes the silver, taking it off the market permanently.
It's basic economics, really.
I'd always assumed such "consumed" materials were replenished by supernatural means. I mean, it's already a universe where spontaneous generation is not only real, it is good hard
science. So, if living organisms can just pop out of nothing, why not minerals too?
Perhaps that same process is what produces living dungeons.
For my
Jewel of the Desert game, we don't bother with denominations smaller than "dinars," which I treat as equivalent to GP. A week's manual labor earns a salary of 10 dinars, which is plenty to live a modest life on (rent a cheap apartment, buy food and clothing for a small family, and have a little to spend on something nice now and then). Given the party regularly gets paid in 50+ dinars for their services, anything smaller would be irrelevant to them. 100 dinars is a good floor price for basic magic items; potions often cost less but, naturally, are consumable. Rare magical ingredients or powerful stuff imported from Jinnistan tend to be in the 50-200 dinar range.
The most expensive thing the party has ever handled was the Desert Rose Ruby, which was valued at over 10,000 dinars for insurance purposes (which, I'm fairly sure, is more than the party has ever collectively earned). They helped officiate its sale from its former owners (who had been on very hard times) to one of the richest men in the city; he paid a pretty penny for it, which knocked him down a peg in the rankings of the richest people in the city, falling from "second place possibly tied for first" to something like 4th or 5th, but he's been working to recoup that.
So, more or less, anyone rolling in the 100s of dinars a year for income would be comparable to a low-six-figures; anyone dealing with multiple thousands of dinars a year is a millionaire; and the sultana, whose net worth of property and treasury and art and such is probably in the low 100k dinar range and might be an effective billionaire if she were a private citizen.