A quick search shows that your Nikka 20 is available for 16.5 k retail. I couldn't find it on the Japan Duty Free site.

Another example Suntory Hibiki 17 is 8000 yen at the airport but 7500 yen (incl the 8% tax) retail.

But sometimes one is surprised by the range of retail. There is this hole in the wall liquor shop near Labi in Shibuya that sold Otard VSOP cognac for like 2600 yen. At a discount shop like Yamaya it probably costs in the high 3000s. Could easily exceed 4000 at Tokyu.