Not counting the foundation, such as in the FIP, we have the following costs for a starter home:

Size:

20' wide by 34 normal size, but add 20'x42 for the one with thermal battery.

Lowest Cost Option

$20k rock bottom options, no cabinets included. Build them when you can afford them.

20'x34' consists of effectively 2 16'x20' sections, 20' trusses on the base. For 34 feet we have 18 trusses at $80 a pop. With roof we have 36 trusses. $2880
Insul r38 = 1400 sf = $1820
Insul white foam $1k [1]
FRP - $600.
Floor and roof - 1400/32*21 [2] - $920
Doors and windows- 3000
Interior doors - 7 - $700 [3]
28 exterior frame modules - $17 OSB + $32 metal at $1/sf. + 4*7.5 lumber [4] = $80 per exterior module with metal. $2240.
Drywall is $9 [5]. 28 ext + 24 int + 22 ceiling = 74 sheets. Sheet! that is a lot. $700. $10k below.
12 interior wall modules - framing - $4*4=$16 ea *12 = $200 in lumber.
Exterior insulation - 60 cents/sf 9'*108' perimeter = $600
Interior insul 0.3 per sf [6] $130
Vanities - $600
Electrical - $1.5k
Plumbing - $1000
Service entrance and - $200
Tubs - $1500. Or go chape -$1k with base. [7]
Toilets - $200
Heat pump - $1600. Can save $1k by going down to 20F.
Stove/cooker - $1000
Flooring - $1.5/sf $1050 [8]

$20k House