I don't know how much a game developer makes in Japan, so I'm sorry I can't help you on that one. One of my friends works for the Playstation division of Sony (although he designs semi-conductors rather than software) but he might be able to find out what the pay rates are like for Sonys own in-house developers. If no-one else can help I'll ask him when I next see him.
As far as opportunities in Japan are concerned, business here has rather a fierce reputation for being rather closed to outsiders (which sadly is often true in most cases). The visa requirements can also be quite strict. I think your best chance is to try and find a job with a Japanese company that has a European division (perhaps Sony, Sega, Nintendo or other big developers would be a good place to start). This would almost definitely bring you into contact with the Japanese parent company. I once knew someone who worked for Sega of Europe and she regularly came to Japan for business purposes. You could then maybe try to arrange a transfer to Japan.
Companies like Sony also often offer optional free courses in Japanese to their employees overseas which would help (you'll more than likely need at least a moderate grasp of the language).
You mentioned in your original message that you are already a game developer. Do you already have experience and work for a non-Japanese game company?
maybe I can help you. There is this guy called Greg, who is and American (I guess) game developer. He lives in Japan and works for Sega (at least he used to).
Maybe you could contact him: http://greggman.com
His site is quite famous, you can find lots of information about Japan and society there.
By the way, I'm on my way to start a career in game programming (not yet decided, but it's a real plan) and I wonder if I could make a live in Japan as a game developer like you.