Spoiler Alert: This comment includes spoilers from Ed Greenwood's Death Masks.
K.o. is misremembering what they read, based on their answer.
The following is a quote from Timeless (chapter 8) about the incident in question:
Regis did some quick calculations and tried not to gasp. A long ton, a tonne, of platinum would be more than a hundred thousand coins, valued at ten gold pieces each! No wonder Clan Stoneshaft had come south with an army guarding their carts. The only wonder was why a horde of dragons hadn’t sniffed out such a treasure and descended upon them.
And where had a clan of dwarves Regis had only barely heard of come up with that kind of a haul? Bruenor, the most powerful dwarf king in the north—at least the north, he acknowledged—would have to empty the treasuries of both Mithral Hall and Gauntlgrym, and likely call in a favor from Citadel Adbar, to approach that fortune.
For the sake of those who are not mathematically inclined, there are 2,240 lbs in a tonne, and 50 pp in a pound. That means the transaction involved around 1,120,000 gp worth of pp.
As you can see from the full quote, Salvatore is saying that this is hardly usual for even wealthy individuals. In fact, it's hardly usual for ultrawealthy individuals. By the sounds of it, Bruenor might not even have a full 1,000,000 gp between two dwarven kingdoms—Gauntlgrym and Mithral Hall. Regis, who is well acquainted with how fantastically wealthy his friend is, almost gasps when he grasps the sum in question.
But what about non-kings? Well, in Death Masks by Ed Greenwood, the 1% of the 1% of Waterdeep can outbid the other 99% of the 1% by a significant factor. One especially wealthy individual bankrolls the assassinations of several of Waterdeep's Masked Lords. The bounty for such an individual? Well, it starts off at 100,000 gp, with a 10,000 gp down payment. The other 90,000 gp was available soon after, but then subsequent payments had to be in gems.
And a little later in the same book, Mirt offers 100,000 gp to resurrect someone—an offer he ups to 200,000 gp when 100k isn't enough.
Now, it would be a fair point to ask, then, how some other occasions exist where someone might be expected to drop a few million gp in the Forgotten Realms. For example, to create a new seat in one of the Moonsea city councils takes 2,000,000 gp.
The answer to that goes back to how values were different in previous editions. While much of the equipment section for 5th Edition was pretty much copied and pasted from 3rd Edition (the "Common Trade Goods" and "Food, Drink, and Lodging" tables are identical and only a few of the weapons have different costs), in 2nd Edition things were quite different. Rather than a bottle of good wine being 10 gp, you could get a tun (250 gallons) of good wine for 20 gp. Nice, sure, but... a silk jacket was 80 gp (instead of 15 gp for a whole 'fine' outfit of clothes in 5E), a large tent was 25 gp instead of 2 gp, full plate armour was 4,000-10,000 gp instead of 1,500 gp, and even a poor lock was 20 gp instead of a decent one for 10 gp.
In other words, to be rich in the edition where you could buy a seat on the city council for 2,000,000 meant that you might possibly be able to afford that.
In short, the D&D economy is screwed up, always has been and always will be. There is maybe a little more consistency now that the writing roster for the Realms has been somewhat culled, but at the end of the day the concept of 'rich' is ephemeral and based entirely on the DM's idea of what it should be.
But just for the sake of it, here's my suggestion:
Income Class ... Liquid Wealth
The Bottom 99% ... Anything up to 100 gp
"Rich" ... 100 - 25,000 gp
"Ultra rich" ... 25,000 - 100,000 gp
"Small nation" ... 100,000 gp - 500,000 gp
"Large nation" ... 500,000 gp +
This is just liquid wealth, remember. Net worth is a whole other equation. It could be anywhere from 10 to 10,000 times more than what your liquid assets are. If you own an inn with five rooms, for example, your liquid assets are probably the 112 gp you got this month from renting out the rooms at 8 sp each, but your net worth includes the food and wine stores, the lot in town, the building itself, etc. You could be looking at 5,000 gp or more. In a city like Waterdeep, where land is at a premium (if the city's population really is 2 million, my rough estimate of the city's density is about 270,000 people per square kilometer... the most densely populated city in our world is 41,515 per square kilometer!) you would be looking at a runaway housing bubble. A single acre might be worth more than a normal average Joe could afford in their entire life.
But I digress... the wealthy are as wealthy as the DM wants them to be. That's the most that can be said on this.