Not a good rule of thumb I am afraid. Take for example, Laurent Perrier 2004.
US$ is the currency I used in below comparison.
LCBO price $69.80
Wassy's in Osaka (Well known wine merchant here, highly typical) $90.89
Checking prices is most easily done via wine-searcher. There is very little here that is not cheaper esewhere when discussing imported highly taxed items. Wine and shoes come to mind. :-)
Rakuten and Amazon Prime have done well for me.
Take a bottle of Ruinart Brut Rose
LCBO unavailable
7440 yen on Rakuten including taxes and shipping. This comes to my hotel room, waiting for me and goes back on the plane to ICN. Amazon is 7880 yen and comes Prime within a day in Tokyo. Can definitely live with $62-66 Ruinart rose!
Wine searcher has one for 45 Euros (very good deal? I don't know what they retail for in Europe) but then I would have to pay 55 EUR shipping and possibly get it seized by Korean customs.
Don't get a chance to go to France and fill my luggage in the Motherland but I'm going off-topic here. You Japanese vets would know more than me
.