Critical Mass for Economically Viable Permanent Demiplanes?

[Dr. Yes]

Skyscrapers are basically the coolest structures that regular people will ever have access to. They're these enormous monuments to a society's engineering prowess, made possible only by large numbers of people wanting to set up shop in the same location at once. Property values have to be high enough to support the cost of building a tower, otherwise the thing doesn't get made or people don't move in.

That got me thinking that the Create Demiplane line of spells is the obvious magical means of solving the same problem. Five dozen merchants want to set up shop in the city's main square, and they're willing to pay a premium for it? Great! They can each set up on a 5'x5' square of wall that leads into their own private demiplane. The thing is, making a demiplane permanent is expensive. The Pathfinder (my group's go-to system) SRD says it costs 17,500 gp---three and a half times the price of a large house.

How densely populated do you think an area would have to be to make demiplane "buildings" in the city center economically viable?

[Deepbluediver]

It's not just the cost; you need to have a city large enough to attract the kind of magical talent that makes this thing possible, and where magic is common enough you won't drive the local populous into a panic when you pervert the physical laws of nature. Also, high end merchants probably cater to a high-end cliental; so you want a fairly large and active noble class or upper crust society.

I would say that it should be a city of 75,000, at minimum. That's not big by today's standards, but pre year-0, mostly it was only the capitals of empires that grew that large. They where also more common in warmer climates where growing food and suriviving through the winter was easier. London didn't reach 75,000+ until approx. the 1400's, or later.