I was really sold on this idea for a while, looking at the prices of Rocio Romero's LV... but then I read a few articles where people detailed their experiences actually buying and building it and like you said, 250-300/sqft and more.
It's similar to the shipper container trend that has accompanied the tiny house movement - its actually more expensive than the alternative, but seems like it could be a more economical option. (I'm referring to shipping container homes that need to be insulated and don't need super beefy locks, in that case they might be advantageous)
Not that I like stick construction very much. It's just the cheapest and easiest (after the balloon inflated concrete dome houses of the yesteryear - google monolithic airforms) for most climates. Living up here in the frozen tundra of Minneapolis I want to build, not buy, and I want to use SIPs. They're not cheap either, but at least you aren't paying for designer design - not to knock what cool architects like Romero are doing, but most people looking at their affordability-minded designs would probably be better off downloading some free blueprints at freegreen.com and having a trusted local architect revise them for the build site, then getting a trusted local GC to execute.
That's what I'm planning on doing anyways. I don't like the idea of buying an existing house - older construction is full of the half ass fixes done by people over the years and newer homes are mostly stick built cookie cutter designs built by a builder who never factored any site considerations into the design and could have very well cut corners to keep their margins profitable.
So, to sum it up, what I've come up with is freegreen.com - their plans seem to have different tiers of insulation so you can easily customize it to your climate. I'm not in architecture at all, I build furniture for fun and working the packaging industry so take what I say with a grain of salt. Best of luck.