What would be the best way to determine how much wealth nobles /merchants should have based on settlement, trying to find the upper limit that someone of high social status would possibly have including homes.
It depends on how rich we're talking about, and on what we take the buying power of the gold piece to be. Looking through prices, it seems like a gold piece is worth somewhere between $100-$600 (american currency, circa 2016):
Hat: 1sp to 50gp (A cheap hat is around 10$, suggesting 1gp = 100$?)
Cow: 10gp (A cow costs around $1000, suggesting 1gp = $100?)
Doctor: 1gp (An average doctor's visit costs around $200, suggesting 1gp = $200?)
Peasant's clothes: 1sp (A cheap set of clothes is around $50, suggesting 1gp = $500?)
Hireling, untrained: 1sp/day (Minimum wage is $58/day, suggesting 1gp = $600?)
So let's say that 1gp = $250.
To be in the 90th percentile (by net worth (USA 2013)), your net worth needs to be around $940,000, or 3,760gp. (That includes everything: property, investments, and so on; so not all of this would be wealth an adventurer could find lying around.) We might call these people well-off.
To be in the 99th percentile, your net worth needs to be around $7.8 million, or 31,200gp. We might call these people rich.
To be in the 99.9th percentile, your net worth needs to be around 30.6 million, or 122,400gp. We might call these people very rich.
Things get weird when you have nobles. They might have inherited a centuries old castle that was built wing by wing over generations.
Also, when considering the value of real estate, things get even wonkier. The game likely only considers the value to construct, generally, but the value can move around massively due to the key three keys of real estate: location, location, LOCATION
A castle on the border or along a major trade route can be extremely valuable. But a castle where you have to go 100 miles to even get to the smaller, no horse town.... yeah. You might not even be able to pay people to take it.
And then there are nobles that borrow way too much money and don't really do much to repay it.
It can be hard to determine how much wealth the person actually has versus what he is fronting. This is particularly true for the kind of people that PC's might end up facing (who overindulge, potentailly hiring kidnappers for 'reasons' that the party has to stop, or seek to follow some twisted scheme to pilfer funds to fight back creditors; a classic example of both of those would be the noble trying to force a marriage to get the girl's body and family wealth).
Overall, due to stealing, lies, extortion, etc: enemies often have more wealth than the tables. Don't sweat it, and only focus on the parts that actually matter for balance: insure that equipment doesn't go over the tables (since a level 1 party should not be facing 10 guys with +5 vorpal blades). Otherwise, just leave the background stuff uncalculated and just say that they stole the money and then squandered it in various ways (wine, women, food, etc.)
As real life can attest there really is no upper limit on wealth. I would assume in a world with long lived races, outright immortality, and revival magic any limit would be even more unlimited
Don't forget "landless nobles" were and are a thing.
Wealth is not automatically tied to a noble title. Social opportunity generally is but not always wealth.
and even landed nobles can be poor if they have little lands, and/or poor lands and thus don't get much revenue.
If you want a rich noble, I know the cost of living for a wealthy person is 100 gp a month and 1,000 gp per months for someone that lives extravagantly. I imagine that's a good place to start from if a nobleman can spend 1,000 gp and still have a lot of money left over.