One has no idea what long term future inflation and productivity gains will be, so unit costs are based on current (Earth) prices and costs unless there are specific reasons otherwise, as is the case with air and water.
A key assumption is that if the habitat is in deep space a strong magnetic field is generated around each cylinder to deflect solar wind and cosmic rays. The water jacket and plastic shell assumed are secondary protection, similar to the absorptive role of the atmosphere on Earth. If the habitat is in Low Earth Orbit, no magnetic field would be required.
Steel: 10mm plate for the outer shell. I have assumed a price similar to current levels on Earth, given that there is free metallic iron on the Moon but processing costs to steel plate may be higher, so $500/ton. For the annular cylinder model, I have also assumed steel for the “roof” shell, as it is the cheapest option. This needs to be airtight, but otherwise no structural load or radiation protection (it is entirely inside the cylinder) so have assumed 5mm plate (typical hot rolled steel sheet is 2-3mm gauge).
Plastic: 20mm plate assumed for inner shell, pretty robust but it has to bear the mass of a metre or more of soil. Current market price for high density polyethylene assumed, $1400/ton.
Water: a 30cm jacket between outer and inner shells (radiation protection); 25% by weight added to dry soil (typical value for agricultural soil); another 10% for other uses such as ponds. The cost of $200/ton is high as it needs to be sourced and transported from Mars or Ceres. It may be cheaper processed from asteroid mining, but as previously mentioned the volumes of water available may be rather limited.
Soil: lunar regolith. Just scoop it up, it’s nearby, so should be quite cheap, $40/ton assumed. 1 metre average depth assumed. There is still an awful lot of it needed, however…
Air: I have assumed 80% of sea level air pressure on Earth ( that is, 0.8 bar) which is like living at 2000 metres altitude, which doesn’t cause people problems (at the 4000m height of Tibet or the Bolivian altiplano it is only 60% of sea level, yet that does cause altitude sickness). Cheap ($20/ton), collected by skimming through the Earth’s atmosphere.
Then there is the cost of assembling the habitat. Yes, it is in space, not a great environment, but you are assembling fairly simple components (presumably using robots) and filling with soil, then rotating as you build to 1g (with such weight, acceleration would be very slow, but that does not matter). A 50% markup on raw material costs should suffice. Then I have added 200K per inhabitant to cover installed capital such as housing, living infrastructure, power through solar cells etc. – essentially US value of capital per person less the value of land.
How much does it cost?
Costs per head are acceptable, at high density