Rates of return on nebulous investments are at the heart of figuring out finance, new growth, and how we should reward entrepreneurs and patent-holders.
Tyler Cowen has read Noel Maurer and Carlos Yu’s The Big Ditch: How America Took, Built, Ran, and Ultimately Gave Away the Panama Canal. He notes the authors estimate:
… Aa social rate of return of nine percent for the first two decades of the canal's existence and they do include the costs of defending it.
That return would be real, but it corresponds to nominal rates of 9% in the 20’s (much worse than stock) and 7% in the 30’s (much better than stocks).
I tend to doubt you could raise money these days for a physical project of this scale that would only return 9%.