I 
Yen
This is a little bit about money. Earnings for different types of teachers and a little something on living expenses.
JET ALT's get 300,000 yen a month, private ALT’s range from 210,000 to 300,000 with average being 250,000 and eikaiwas vary a lot depending on the number of students and hours put in but for the most part starting pay for full time work should be 240,000 to 276,000.
Contract completion bonuses, most private ALT's and eikaiwa teachers, range from 50,000 to 100,000 on average for one year, but I once saw one for 600,000 on completion of two years. JET ALT's I believe have their return flight paid for and take home the money they paid into the pension program as a bonus.
Extra work taken outside your company will earn you around 2,000 to 5,000 an hour. Sometimes you can get special gigs or group lessons that earn you a flat fee like 10,000 for two or three hours. These are your average rates too for freelance teachers who give lessons legal or illegally on their own. Think of GABA who pay 2,000 to 3,000 an hour to their instructors who do one on one lessons. University and upper tier high schools pay 400,000 to 500,000 per month to start out. The best postions go to those who are published. I don't know what sort of bonus they receive, but consider that standard Japanese yearly bonuses average two months pay.
Out of your pay, unless you are freelancing illegally, comes income tax which seems to be a flat 10 percent. JET ALT's pay into the pension program which you will get back something like 60 to 70%. This usually adds up to 40,000 Yen out of your pay in taxes.
Bills- Keitai (cell phone) 5,000 to 3,000 Gas 5,000 to 3,000 Water 1,000 Electricity 5,000 to 3,000 Of course all of these are based on usage and vary a lot with the time of year.
Rent is one thing that varies on location but average small apartment is 40,000 to 55,000 a month. The closer to Tokyo the more expensive your apartment will be. Also there is key money. Sometimes you get key money back but never all of it if you go through a realtor because they have to eat just like everyone else and have fees that they charge. Some companies provide you with a housing supplement, but this is rare. Also you need to think about internet and appliances. A lot of Japanese homes and apartments are not wired for the net so be sure to ask your employer if your place will be. And try to get the minimums provided by your employer like a futon and pillow, a bike, some pots and pans, fridge, washer and a microwave or toaster oven.
A car is probably one of the most expensive things you can own in Japan. There are businesses that will lease you a vehicle if you plan on only doing a year or two. Car expenses include 1) the cost of your car 2) the gas you spend driving around 3) driving on toll roads 4) vehicle tax 5) shaken which is an inspection and repair service done every two years 6) getting rid of your car when you leave or it breaks down 7) parking (you pay like 3,000 to 5,000 a month to park at your own apartment). If you want a car and find a job that says you are going to be driving around a lot, take it because this means they will give you a car and pay for half of this stuff. To get an idea of car costs check out the infozone on jetsetjapan.com in the links I will post below.
School Related
Every now and then, you have to pay out a little bit to do your job. Here are some of the expenses of being an ALT.
Kyushoku (Lunch)-Probably one of the cheapest meals you will eat in Japan and most likely one of the worst from time to time. Price-4,000 yen a month
Enkai-Quarterly or so booze parties for the staff to get together and unwind. Why do they do this on nights when you have school the next day, I have no clue. Price-3,000 to 5,000 yen per enkai
Gifts-You go somewhere and they expect a souvenir (omiagei) or someone gets sick and they collect money.