How much money do I need to make a decent living in Japan?
At least 100,000 yen per month just to scrape by. For a young single dude with no wife or kids to support, 200,000 yen should be the bottom line for comfort. Japan is an expensive country to live in, but even more so if you insist on living in the same way you did in America or Europe. (Imagine how much it will cost to live in a traditional Japanese style house and eat authentic Japanese food in Paris.) The more you blend in, the less expensive it will be. If you really get broke, check out the Surviving Like a Bastard section of this site.