The major expense is not the walls of the house. It is what goes inside, under, and over it: kitchens, baths, flooring, electric, plumbing, land, foundations, roofing, etc.
What intrigued me was that for those in the third world these types of housing could be built by families and could provide very inexpensive a safe shelters.
"Earthships" - I think the original idea was to do something about the thousands and thousands of tires that were just being thrown away when earthships were first designed in the late 70s, or early 80s. I believe they've since come up with all sorts of ways to recycle old tires today.
"What intrigued me was that for those in the third world these types of housing could be built by families and could provide very inexpensive a safe shelters."
There was talk about using straw bales to quickly provide shelter in disaster situations, but I don't think anything ever came of it.