Buying the shell and building yourself is the only way this makes sense to me. (Financially)
Once built - the initial investment from doing everything on your own - say about $40,000 will increase in value to more around $70,000 at current market value. Trouble is, you can probably only live in it for 5 years or so before the depreciation starts to catch up.
I read that the average time people spend living in their tiny homes is about 18 months.
They can start out small and add on as they go; depends on zoning laws and how big a rip-off the local govts. are.
Back in the 'good old days' buying house kits from companies like Sears was pretty common from the late 19th century on, and people would start small and add to them as the family grew or they needed more room.
Being poor was also a lot different in the past as well; even in a city like Chicago or New York City a family could keep a cow or two, chickens, pigs, whatever, behind the house as well. Most can't do that any more; can't keep a milk cow out on the patio or balcony of your apartment without your neighbors or the management running and sniveling to the cops.
I can only do all that because when I bought my parcel of 20 acres it was already zoned agricultural and grandfathered in by the state and the Burbs grew out around me, so the developers and their city council cronies can't do squat about it.