I'm not sure how you can conclude that for any infrastructure project, complexity scales sublinearly. This feels true in software, but not real world matters IMO.

Think of all of the politics & hard choices involved, for instance, in displacing people vs. bending over backwards to accommodate them with the California high speed rail project.

If anything, complexity probably increases exponentially as you have to consider ever more permutations of possible project.

I agree, and think that this should be incorporated into the cost.

Ie, a project twice as complex should be 5x the cost due to the additional management, comms overhead, permutations and potential risk.

Said another way the curve is logarithmic and there's a theoretical upper bound on project complexity.

Total project complexity isn’t the relevant metric here, a project with 100x the budget has more people managing it any many decisions don’t make it to the top.

Also, if a 2x as complex a thing takes 10x the budget then that scaling is already part of the budget figure.