Comment by Analemma_

> unless you think an $8 Big Mac is twice as productive to the economy than a $4 Big Mac.

It literally straightforwardly is. It provides the same number of calories, to a worker who is twice as productive in the $8-per-big-Mac city as the $4-per-Big-Mac city. Differences in per-capita productivity between supercities and hinterlands are vast.