Cost to build Gerard K. O'Neill's "Island Three"

Pat Flannery wrote:

You've got to give the Romans credit for coming up with one thing that
completely changed the world...they invented concrete.

I guess that's balanced by inventing the civil service structure. Throw
in the invention of the republic and they are back in the posivite for
me.

...and not only that, but really top-notch concrete that's lasted intact
for 2,000 years, and even concrete that would solidify underwater.
That beat the hell out of carving building pillars out of solid stone,
or trying to make things out of stacked bricks like a huge scaled-up
Lego set.

They did well with piled up stones as well, they just weren't inventors
in that realm.