Back in 1983 when I first came to Japan on a working holiday visa, I was a backpacker with the experience of having hitchhiked half way around the world (I never made it past Turkey, what with the Iranian crisis back then). So arriving in Japan I was determined to live as cheaply as possible and use the savings to pay for language lessons and travel. Now, 25 years later, I'm pleased to be able to report that backpackers arriving in Japan these days don't pay that much more than I was back then - although of course salaries haven't moved up that much either!
Low-budget living is not for families, and barely for traveling couples. If you're single, though, all you really need is a roof over your head, some nutritious meals, and some occasional social activity to give you a reason to be here in the first place. To provide a bit more realism, I also included some Japanese lessons, just in case you came back with a Japanese partner - a common event I find.
The biggest cost for most newly arrived travelers is finding reasonably priced lodgings. Luckily, what was a risky roll of the dice back in the eighties is now a reliable form of accommodation. I speak of the much-maligned "Gaijin House". These consist of an older style house in the suburbs that has been purchased by a company or individual landlord, and then shared between 4-6 people.
Older houses are preferred, as they have sufficient bedrooms and storage space to accommodate a larger number of people. My first gaijin house had 5 bedrooms, and 2 people shared each one, and during weekends, another 4-5 people would "crash" on the living room floor in their sleeping bags downstairs. The shower was a dirt floor unofficial addition to the back of the house, and the kitchen overflowed with people and uneaten food. I paid JPY50K a month for a room I shared with another backpacker.
These days the conditions are much better, and most places will offer you a clean individual room. If you're looking for a reliable landlord, and not coincidentally the biggest, take a look at www.sakura-house.com. For some choice on pricing and locations, several other Gaijin Houses with widespread presence in Tokyo are Bamboo House (www.bamboo-house.com) and Tohto House (http://www.t-guesthouse.jp/). Rents for most gaijin houses vary according to distance from the city and whether you have your own room - i.e., they're predicated on the cost of real estate. Basically you can count on a single room costing around JPY70K-JPY90K if the location is within 30 minutes of town.
A. One-time costs:
1. Shopping bicycle - JPY20,000
2. Room deposit, around 6 tsubo (20 sq. m.), 30 minutes away from center of city - JPY70K-JPY90K
3. Basic furniture - supplied, but you may need to buy some bedding and kitchen gear - JPY50,000
TOTAL: JPY140K - JPY160K
1. Room rent, Shinjuku or Shibuya areas - JPY80K/month
2. Food for 1, few or no foreign items - JPY70K/month
3. Drinking money, go weekly with friends, tabehoudai - JPY20K/month
4. Japanese language school, 3 days a week - JPY50K/month
5. Misc. and travel (get a monthly travel pass before you come to Japan) - JPY50K/month
Total: JPY270K/month => JPY3,240,000 annually
Clearly if you're not going to do Japanese language lessons, nor socialize (I guess you could be studying Aikido in some aesthetic temple in the countryside?), then you could get the monthly bill down even further. However, I have constructed the JPY270K scenario on what people are likely to be doing if they're earning the basic minimum amount that Japanese immigration wants to see you on if you’re living in Japan as a long-term resident.