Rezz,
Have you considered exporting a car when (if) you leave Japan? The laws on importing cars in to Australia say that you have to have owned the car for 6 months and have is registered. Then you throw it in a crate...save your 12,000 Yen for a few months then buy a whole heap of cheap 2nd parts...throw them in a crate and come home...fit some to the car and sell the rest for a tasty profit.
Your thoughts? (this is what I was planning on doing)