How much does it cost to naturalize?
Believe it or not: ¥0.
Compare this to the ¥8,000 you need to apply for 在留の資格 ([alien] residency status) of 永住者 (permanent resident) and the additional ¥3,000/¥6,000 you'll probably need for a 再入国許可 ([Japan] re-entry permit) depending on if you ever have to travel outside of Japan once of more times more every three years.
Of course, this is the do-it-yourself price. You may find that you have to hire legal help to prepare the paperwork, or pay for Japanese translations if you're unable or unwilling to do the work yourself.
And it doesn't count the cost of obtaining some of the paperwork, which is not free. In Japan right now, the service fee for getting the following documents is usually as follows:
- 登録原票記載事項証明書 (certificate of registered alien matters): ¥300
- 戸籍謄本 (complete copy of family register): ¥450
- 住民票 (resident registration card): ¥300
