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?