Tim Ferris has two posts on how to travel in Tokyo for reasonable rates. Of all the things we did love about living in Japan, the cost of living was not one of them. We had it pretty easy, since we're DINKs (Dual Income, No Kids) and had the benefit of the cost of living allowance (COLA) that we got through the military. Still, everything was more expensive there in Japan.
Gas was upwards of $5 a gallon.
Bottle of beer: $6-8.00 (depending on type)
Rent: 190,00 Yen (anywhere from $1500 - $2000, depending on the yen rate)
Dinner out: Depends on the restaurant, but if we went out or up to Tokyo, we would usually spend upward of $120 for both of us.
Due to the safe nature of Japan, we regularly went out with 30,000 yen in our pockets. In fact, if going to Tokyo, we rarely left with less than that, because it was usually pretty inconvenient to find an ATM. Very few ATMs we found worked with US cards. A tip if you are traveling there, look for a post office, as they usually have a postal savings ATM, which for some reason works.
Take a look at the posts, especially if you are preparing to go for a visit, or if you are already living there (big ups to my peeps there at Zama!). Good stuff...