Well, times have changed then. I apologize for not being up to date on this
Price gouging certainly was a thing several years ago with lesser chains like Subway, and it still is on everyday items like toilet paper and peanut butter. I wonder how much diaper prices vary (to go back to that Swedish tourist study).
FYI, a Big Mac at DEN costs $4.95 so 25% above the standard US price.
I recall arriving at out empty house about 4 years ago in the evening after a 14 hour flight. I was only aware of the convenience stores (which I detest) and the Kinokuniya in the basement of Tokyu Honten in Shibuya. I headed down there to buy some food for breakfast and some toilet paper, which was frightfully expensive. I also thought it was lousy that the real estate company did not provide a few rolls of toilet paper in the house.
I was not aware that there was a Donki across the street (could not read katakana) where I could buy toilet paper for probably 1/5th the price. I was aware of Don Quixote having visited one in Hawaii.
So in the same neighborhood you have a huge price differential for something basic like toilet paper. The McDonald's around the corner sold hamburgers for 100 yen (at the time) so no location related pricing, and I am guessing the same for the Yoshinoyas, Sukiyas and the many other chains in the area. And the Hermes handbags in Tokyu Honten are probably the same price all across Tokyo. The bigger question is whether you could find them out in Hachioji, Kunitachi or Saitama.