I think in my view, the ranking would be something like this... but all depending on currency fluctuations.
Europe - UK is cheaper than Paris at the moment apparently because of the poor GBP, then within Europe, Italy is very competitve with Paris given it's slightly higher detax rate, so I actually rate it UK, Italy, France, then rest of western Europe (eg Germany, Spain etc). Eastern Europe is quite expensive.
Then probably Hong Kong after that, and then the US I'd say.
Then Australia (due to the strong AUD, prices haven't been increasing that much over here recently) and some other Asian countries like Singapore and Taiwan should be relatively close. Then perhaps China.
Japan is on the high end. I'm not sure if there's any places more expensive than Japan but on the other hand, you WILL see the MOST number of Birkins on shelves (one time, at a store we saw 6 Birkins on shelf).