Yeah, I ran across that SIP cabin thing and it piqued my interest too. I can do all the mechanical, final roofing etc myself, basically just the cost of materials on top of the cost of the dried in structure. Thing is, I'd need to make it a full time endeavour for about 6-8 months.
EDIT: Here's what the $45,000 price includes:
http://www.octagonhomes.com/whatsincluded/
"Everything to complete the exterior of the home is included with the shell except finished roofing materials decks or porches.
Delivery and erection within 75 miles of Ellijay, Georgia (delivery charges may apply for longer distances).
1st and 2nd Floor system 2x8 12" On Center, with plywood glued and nailed
Walls 2x4 16" On Center with O.S.B. and house wrap installed. White Pine chink log siding and windows installed, Exterior Doors are also included
Roof System with 2x8 16" On Center, finished gable overhangs, decking installed with felt paper
A small amount of exterior trim, fascias, and siding to be installed by home owner (supplied by Connor Construction)"
http://www.octagonhomes.com/whatsnotincluded/
"Foundation
Finished roofing materials (shingles or metal)
Decks or porches
Stairs and interior studding
Plumbing and electrical
Between studding insulation
Interior walls ceiling and floor coverings
Cabinets
Heating and air"
It also appears this is NOT a SIP system. i.e. SIP is EPS foam bonded between two sections of OSB. This is a panelized stud home. That's OSB nailed to 2x4's like conventional construction.
Foundation costs vary with the property, depends on just what you want. Standing seam metal roofing (materials only) would be in the area of $3500.00, shingles would be cheaper.
SvRex is not kidding about this being a $200K house when finished (figuring the average cost of a foundation, lot, interior finishing, mechanical etc) but an individual willing to bust azz could bring it in considerably lower.