Just as an example - I had a look on amazon.com and amazon.co.jp -
8gb nano on amazon in US - $249.00
8gb nano on amazon japan - Y29,800 -
which at today's exchange rate is about the same. Plus in a shop you would save 5% tax.
I'm not sure if the electric shop outside of security at NRT sells ipods or if there is any electric shops inside security.