You would need to build a foundation and choose materials for the exterior and interior, provide insulation and wiring and plumbing (if desired). Though you could just leave it as a steel stucture. The yurt can be assembled in one day and can be taken down and moved if needed.
The cost of the metal yurt is 10K
Grain Bin Home
The home around 10K but there is a big discount for non-profits who are purchasing them.
SIPs Prefabricated Dome
Eco Built Systems makes these really promising little MgO based modular homes. The walls are SIPs made of MgO, basalt, glass fibers and mineral foam. They claim they give off no VOCs. Because these are prefabricated homes they reduce constuction time compared with a tiny home. They claim cutting down on
constuction time by 40%.
Did not hear back from them via email regarding the cost so not sure if these are available right now.
The plans for the treehouse are $450 and includes 3 hour consultation. The metal parts are $2000-2500 and the wood would come to around 5-10k. Not including materials for wiring and plumbing.
The 12x12 which has room for kitchen and bathroom is $2708 which includes the basic kit plus a fireplace thimble, Super Span Roof Panels color upgrade, and R25 insulation.
This is a simple, mostly metal kit that you could then customise to be chemical-free on the inside.
They are made of fiberglass composite material which the company says does not emit an odour. Some sensitive people say fibreglass needs some time to offgas and others find it ok fairly soon after production.
The domes start at $7,500 for the 14' model.