[pfdietz]

There are no dollars in the laws of physics. It's connected ultimately to productivity of all the activities involved, and there's no obvious upper bound to productivity.

[WalterBright]

There's the rocket equation.

https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation

[pfdietz]

Which doesn't have dollars anywhere in it.

[WalterBright]

The point of the rocket equation is there are hard physical limits on what rockets can do, and dollars won't change it.

[pfdietz]

That's nice. It doesn't imply a lower bound for costs, just of ratios of costs.