I don't think there's many homeowners or wannabe carpenters on HN...

If you go to a typical Home Depot, at least around here, they have a wide variety of outdoor "garden sheds" on display for all price ranges from $1K to $6K or so. You're talking about one around the $4K size, just a little too small and underengineered to frame a garage door opening.

Then you need to add the additional stuff, insulation, windows, etc. You can add that as you go. Its cheap to frame in a window opening and later install a window.

My grandfather turned a very large garden shed into what amounts to a workshop when I was a kid. I guess a workshop with plumbing is a small house.

I would advise anyone making an outdoor workshop or whatever to do the substantial structural reinforcement needed to install doors at both ends. Not just for convenience of driving cars thru, but the incredible opportunity for shade and ventilation two doors provides. The structural reinforcement required was huge / amazing all I remember was multiple 2x10 beams.. without a cross member its susceptible to folding from wind unless the joints are ridiculously overbuilt, which they were... An a-frame would have been infinitely simpler structurally than trying to make something that looks like a conventional garden fence.

10x10 is half the floor space of this dome.

Yes. The structure I priced out was also only 8 feet tall.

About half the cost -- if I recall -- was doors, windows, and skylights, and the other half was the walls, ceiling, and floor. So you'd expect a structure with roughly double the surface area to be roughly $3000 instead of $2000. Depending on material choices, you would probably end up with a cost of between $2000 and $4000 by the time you put the last coat of paint on it. Again, having not actually followed through I don't have a final bill of materials. I could have over-estimated the cost of something, or forgotten something else entirely.

But the point is the same. Is $2100 really that cheap, when compared to a $4000 stick-frame structure with comparable function, or is it only cheap when compared to a $40,000 house with a bathroom, kitchen, furnace, water heater, and appliances?

And if you hit up your local Recycling yard you can often acquire a number of windows and doors at very economical prices. I once stayed at a house that had an entire wall made of doors of varying styles, salvaged building materials can lend a lot of character to a structure.

You can't plan, receive approval for, build, receive inspections on, and move in to a house for anywhere near as little as $40,000. Try double or triple for a bad one. Perhaps with the exception of a kit-home in nowheresville, NO.

You're more or less right, but I can weasel a technical victory.

The cost to build a house is essentially per square foot. In my area, about $140 / square foot for decent quality. Not the best quality, but good enough. So a small house (1000 sq ft) costs about $140,000 to build, and a large house (4000 sq ft) costs about $560,000 to build. That's parts, labor, permits.

But if you're willing to build what amounts to a studio apartment, you can totally build it for next to nothing. The smallest standard set of plans I found online was 213 square feet. Even adding in the $2500 they're wanting for that one (most standard plans are around $500), that still gives you a price tag of $32,500, which gives you a $7500 margin between the estimated budget and the stated $40,000, for cost overruns and furnishings.

Of course, the point wasn't "You can build a house for $40,000, that's totally cheap", it was "Now, a $2100 dome is 20x cheaper than even a minimal house, which is fantastic, but it's nearly half the price of a $4000 stick-frame structure with identical functionality. Is that cheaper-enough to declare domes the way forward on price alone?"

Bear in mind that a square, straight sided structure will have a higher ratio of usable to unusable floor space than a dome.