Check out these SIP prices! $12-14 sq. ft.!
No studs - No Trusses - No Attic
No attic ventilation or moisture to deal with. Wiring conduits in place.
Of course that is the price for just the shell, but still...
Be sure to click on "Home Plans" and then "Cost Effective Home Building Plans"
These look real good, but I am not sure if they require a center beam and supporting posts. I think most do if they are over a certain size.