$210,000 would strike me as cheap for a permanent above-ground structure of that size, never mind an underground one. Looked at another way, $2100 sounds about right for thirty yards of concrete storm sewer pipe but orders of magnitude off for transit—prefabricated tunnel sections big enough to drive cars through and strong enough to carry however many tons of earth or rubble would not be cheap to make or to move into place, especially if there isn’t an above-ground transport grid to carry them on.
$210,000 would strike me as cheap for a permanent above-ground structure of that size, never mind an underground one. Looked at another way, $2100 sounds about right for thirty yards of concrete storm sewer pipe but orders of magnitude off for transit—prefabricated tunnel sections big enough to drive cars through and strong enough to carry however many tons of earth or rubble would not be cheap to make or to move into place, especially if there isn’t an above-ground transport grid to carry them on.