The value of gold, and a possible rational for why it is so plentiful in D&D

[Tyndmyr]

You can pull a gigaton of gold out of the planet's mantle and not even make a dent in the supply. The issue with D&D isn't the plentiful supply, it's that the stuff has any value at all. It's probably easier to assume that the coins are just a fiat currency like the dollar. It has worth because the government says so.

One solution with historical merit is to assume that the coins have been heavily debased. Rather than being pure gold, they are...lightly goldish coverings for other metals. This has happened very frequently throughout history, and would be honestly pretty easy to pull off with magic anyways, so it being the de factor standard answers a lot of questions.

[Rogar Demonblud]

So a fiat currency.

[Peelee]

One solution with historical merit is to assume that the coins have been heavily debased. Rather than being pure gold, they are...lightly goldish coverings for other metals. This has happened very frequently throughout history, and would be honestly pretty easy to pull off with magic anyways, so it being the de factor standard answers a lot of questions.

Let me tell you about this crown I have....

[Florian]

Apparently there is an asteroid chuck-full of precious metals that if we use today’s prices would be valued at seven quintillion dollars . . . $7,000,000,000,000,000,000!

In a setting where asteroid mining was a thing, then likely the value of precious metals, such as gold, would be greatly deflated.

In a sci-fi setting, such as The Expanse, gold would be worth considerably less than it is currently valued at. I can see people walking around with inch-thick gold necklaces without a care in the world of theft.

In a fantasy setting I can see void dragons traversing the stars, and mining gold for their slumber chambers. I can see them quite puzzled
that these humans keep stealing their bedding. I think that it would be humorous if the gold coins dragons used for bedding had sheep on them (or the equivalent).

In a superhero setting it would take one spaceship, or one Superman to bring the world a nigh-endless supply of precious metals (fixed).

Hm. I think you are overrating what "gold" actually means and why it has any value at all. That might actually come from you being from the U.S.

Let´s use the "silver dollar" as an example: The lowest it can be is "one dollar", being the "face value" stamped on it, else it could be the "trade value" of the silver or the difference in silver trade value between two different currencies.