Wife and I are 67 and have some health problems. We looked at the mini barns, but chose to go with a used RV trailer for a retirement home, placed on our kid's property. Our present home is far too large and will be sold eventually. The RV was built in '88, and needed some work--redid the roof and vents, replaced the fridge and furnace, plus new gas lines, and decorative work. Total cost will be about $2900 complete for 8' x 28' = 228 sq. ft. of space. Note that this cost is ready to live in.
A septic tank and field cost a little over $1500, since the guy was already on the place doing other work. He also leveled a spot for the trailer. Extending the county water line from the kid's house to ours cost another $600 for materials and renting a ditch witch. At this time we put down 4 layers of 4 mil black plastic on the trailer site and covered it with gravel. That may get concrete later, but is fine for now.
A pole framed protective roof is under construction, so maintaining the trailer roof is not an issue. This provides shade to keep that trailer from becoming an easy-bake oven in hot weather. Solar PV will provide electricity, mounted on that roof. We plan to make screen walls for this shelter for bug-free summer use. This will cost about $1,500 using some salvaged materials.
It looks like the total cost will be about $8,000. For that we get a place that is ready to move in, has gas heat and cooking, no electric bill, no sewer bill, and minimal water bill ($20/month). We have plans for buying a used truck body, a reefer unit that will be used for storage for the lawn mower, garden tools, and a small workshop area. That will cost another $1,200 including moving it in. It is 8 ft. x 24 ft., lockable, waterproof aluminum inside and out, and insulated. Including this, the cost is still well under $10K, and people spend a lot more than that on a used car that depreciates as fast as bananas.
Looks like we can live there REALLY cheap. And the kids are right there next door for when we get really old and need some help. Sure beats a nursing home.