I lived there for a year and got paid 80000 yen a month to do so. Considering the dorm I got to stay in was 5000 yen a month and utilities were about 4500, I had 70000ish yen left to travel and eat with. All of this was through the Hiroshima University Study Abroad program (aka HUSA). If you are a student and want an opportunity to live in Japan I recommend this program. It is offered to students from many different countries and they will work with you if you don't know Japanese when you get there. So not being a US citizen is not a problem. Through the University you get a student VISA for one year and there are many opportunities for field trips to high schools and home stays with families. If you have any more questions pm me.
Also, there are a lot of websites out there if you want to teach in Japan. check out http://www.all-about-teaching-english-in-japan.com/teachingjobsabroad.html
The schools mentioned above like Aeon and Nova were popular when I was there, but that was over 5 years ago and things can change.