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?"
