Not as many $$$ as you think
I have some friends who already live off the grid in Ontario (Canada). With substantial insulation (which at time of build is not very expensive) and innovative construction techniques, the actual build cost is certainly less than double, and usually about 30% more than a traditional frame house - depending on the levels of seamless living desired.
I am a big fan of government mandated sell-back programs, where instead of me maintaining batteries, and having no electricity when it is dark etc, I sell electricity to the grid when I have a surplus, and buy it when I have a deficit.
That way, I can save wherever possible, but still not sacrifice lifestyle if I do not wish to.
