I am thinking this would be great for a rental suit, and you never be disturbed by the noise they make. This is the finished project

https://www.youtube.com/watch?v=KO25JYAaJC0

This is the build https://www.youtube.com/watch?v=p1t7r3SKT2o

I did the math 58 sheets (he said 4mm thick, but I think he rounded up), 600 lineal ft of what I think is 2.5 inch box steel (I am within a 1/4 inch) for a grand total of 6500.00 in steel (within10%)

Not bad, I would think the concrete and excavator work would cost more.
I would estimate a two man build of this to take two young guys who know what they are doing 6 weeks, two old guys like me 2 months because there will be a lot of going up and down ladders. The main room is 16x20 feet if you add the area of the tunnel he has a little over 400 sqft