Well we can make a reasonable guess based on The Hobbit film’s shots of the inner rooms of Erebor.
Now judging by the length of the storeroom, very long…
And the width of the storeroom, very wide…
And the depth of the treasure mounds, very deep…
We can safely assume that Erebor had hundreds of trillions if not a quadrillion in its “savings account”. This is made up of endless gold, mounds of precious stones, and the Arkenstone.
In comparison to the stores of Erebor, the US government only has about $500 billion in gold reserves, all of which could fit nice and easy in the storeroom of Erebor. In fact a single one of the large mounds pictured in the movie would be equivalent to those reserves.
Now there is no actual way to tell how rich Erebor actually is. The movies and the book do not elaborate on it. We just know there’s A LOT of treasure.
I will note that the popular estimates are way off and way too conservative. The movie shows piles of gold far and beyond the dragon and the popular internet article only estimates a single pile of treasure based off the size of the dragon. That leaves the vast amount of treasure in that room unaccounted for.
Anyways, Erebor would be the richest nation on earth if it weren’t… you know, fantasy.