My $87.50/sq.ft. positive energy home (with a 10KW solar array, it produces more power than it consumes), which is not an "earth ship," is built like a refrigerator (120 mph wind spec) and thus is largely fireproof. Plus its insurance costs are about half of my conventional home since it's "industrial grade." And, it's made from long-existing (steel-home) technology, with materials that can be made from recycled metal. Finally, it's very low maintenance (concrete and steel, so no termite issues either).
It has cost me $175,000 thus far (compare to this $450,000 positive energy home, and this $499,000 zero-energy home). Actually, it only cost $154,000 because of the $21,000 in tax credits that I took for my solar array.
Here are "external" shots of it. Here are some internal ("Solar Foil Ceiling") shots, along with some internal shots.
But since I'm against such subsidies, we won't use that lowered ($154,000) number here, so divide $175,000 by 3200 sq. ft = $55/sq.ft. (less if you factor in my tax credit). Compare to a typical "greenie" home -- without land cost -- at $550,000 for 2000 sq. ft. or $275/sq.ft. And my building's shell can go up in just a matter of days.
Of course, my number's misleading, because 1600 ft is unheated garage space, and some may insist that per sq.ft. value for garage space should be less.
But my home is designed to permit a lot of living, not to mention working (enough for many a home business), in that garage space.
So it's fair to assign some of the garage space to the net cost per square foot calculation. I think 25% is fair. Assigning 25% of the garage (400 sq.ft.), brings the number to $87.50/sq.ft. ($175,000/2000 sq.ft.).
Note that I'm not including the value of the 920 sq. ft. of concrete, wrap-around porch space.
Also, I'm not including (just to be conservative here) the value the solar adds to the home itself, a point explored by the NYT here.