A friend had a 30x40 pole barn with 10' walls framed up, in the country so no permits or review required. Has a 6'x15' porch on the front, three 36" doors, front, kitchen and master bedroom. Stained concrete floors with PEX in them for future. 2x6 walls, 6" spray foam in walls and roof, A/C is in the attic which is also the return air plenum. Open kitchen and living room. Master bath has a shower, hall bath has a tub, 2 bedrooms, though one doubles as a study/playroom for the grandkids. As I recall, cost him about $68k 6-7 years ago, paid cash for everything. Oldest son is an Electrician, has other friends for septic, A/C and plumbing, basically his costs were all materials.