The problem with the 400,000 a month salary is the huge Japanese tax percentage. 40% of that is easily eaten in taxes, not to mention the 10% sales tax.
We could be talking more just 200,000 a month. There is rent, food, etc... If a guy is paying 30,000 a pop, he can run out of money pretty quickly. Prices in Japan are higher than I've seen any other place in the world. It's a bit crazy.
There are a string of international news articles talking about how only 50% of Japanese actually have sex. A major crisis for Japan. Arguably, part of the problem is how expensive and how much guys are paying for sex.