Do you have to have a lot of money, make a lot of money, or have a really good job to naturalize?

Filling out the paperwork on your income and expenses, in fact, feels a lot like completing a major credit or loan application in Japan. They're concerned not just with how much money you're pulling in, but how you're spending it, how stable your employment and career is, and whether or not you have enough skills (i.e. Japanese language) so that if you were to ever lose your employment, you stand a chance of getting another job in Japan.
Thus, having a well paying job with a large stable company helps your application as it makes calculating your risk profile much easier in a good way.
In particular:
- While they don't provide exact numbers, if you are single with no children/dependents, an income as low as ¥150,000 to ¥200,000 a month from a stable source is adequate.
- Even if you don't have any income to speak of yourself, if you are being reliably provided for by any combination of your spouse, your children, or your family, this is okay as well.
- If you have no income, but you have so much in savings that you can prove you can live off of that exclusively for many years, you can naturalize. (lucky you)
- SPECIAL CASE: If you were formerly Japanese (and not through naturalization) — for example, you were born a Japanese woman and renounced your Japanese nationality for the purpose of marriage and life with a foreigner — it is okay to be on 生活保護 (Japanese welfare).
- If you have a stable job and an adequate income, it is possible to naturalize with zero (¥0) savings.
自己又は生計を一にする配偶者その他の親族の資産又は技能によって生計を営むことができること。
(The total assets for you, your spouse & your relatives, as well as your skills, can provide for your livelihood.)
... will be those who are earning money from lots of small irregular part time jobs or those using 厚生労働省職業安定局ハローワーク (Ministry of Health, Labour and Welfare Ministry's / Employment Security Bureau's "Hello Work" program) like government programs to get employment.