It is possible that same price difference may not continue on as the levels get higher?
With the import/export function Rich Areas are no longer useless. They allow a hex to generate more Supplies than any other hex would otherwise be capable of doing.
I don't know the resource costs for the higher levels, but the Devs obviously do.
It may need some tweaking if default supplies continue to be better value at higher upgrade levels, as that doesn't seem right to me?