I have spent twenty years messing around with cheap ways to build, tried a couple of things and worked on other peoples projects. From this knowledge, and having a best friend that is an architect, I can say the cheapest way is to stick build. I built a metal building and finished out the inside, and by the time I got done it was far more expensive than stick building. Metal buildings need a slab poured and a crane to set the beams. Then you have a shell, but it is uninsulated and you still have a long way to go. If you got a good enough deal, or found a already standing building for cheap, then the economics might make sense. You would still have a house that might be harder to sell than a stick built.
The monolithic domes I have checked out, and went to Texas to look at a few. They are not cheap. You will need a specialized crew for this, and they are hard to find sometimes. First the balloon has to have a very powerful fan that may need to blow for a few days. Never heard the car thing, but that wouldn't work on anything I have ever seen, not even close. You will have to have a shotcrete contractor to spray the concrete, and they better be good because if there are problems, they can very hard to fix. Monolithic did a school and they had repeated problems and ended up scrapping it after it was built. Properly built, they are almost impervious to damage. They have survived documented tornado (near miss at their headquarters) and a well publicized home that has survived hurricanes. They also fired a 30-06 at a dome at close range and it deflected. Again, you have a shell that you will have to finish out, and living in a dome isn't easy. You waste a ton of space because you can't use some areas that are circular. You still have square applicances and furniture. I have relatives that built a dome, and it turned into a bigger deal than you might think.
In my area, I am leaning toward block for one reason. Labor. There are lots of unemployed block layers around that don't speak english and will work for cash very cheap right now. Dry stack appeals to me, and is supposed to be stronger than traditional block construction. Going underground is expensive, period. The groundwork, the insulation against moisture, and other precautions run up the cost fast. I would lean toward partial earth bermed for insulation purposes. The ground is very, very wet where I live and underground means water intrusion.
I have also looked at some other things like building from shipping containers and even using airplane fuselages. The only thing that I could make work from an economics stand point was to buy a older yacht and pull the drivetrain and partially bury up to the deck. It already has a galley and head and you just plug in water and electricity and run a sewer out. Most old wood yachts can be purchased cheaply as the cost of keeping them up has gotten out of control.
These guys are VERY knowledgeable on small build it yourself projects: http://countryplans.com