In terms of lodging, food, and transportation (except taxis), Tokyo is about as expensive as New York or Paris. One good way to hold down expenses is to avoid taxis (they tend to get stuck in traffic anyway) and try the efficient, easy-to-use subway system. Restaurants for locals tend to be less expensive than those for tourists, so instead of going to a restaurant with Western-style food and menus in English, go to places where you can rely on your good old index finger to point to the dish you want, and try food that the Japanese eat.
ATMs at many Japanese banks, stores, and post offices do not accept foreign-issued cash or credit cards except for Visa. Citibank has centrally located branches in most major Japanese cities and ATMs that are open 24 hours. Japan's most progressive bank, UFJ, is a member of the Plus network. Some convenience stores also have cash machines in the Plus network. Post offices have ATMs that accept Visa, MasterCard, American Express, Diners Club, and Cirrus cards. Most bank and all post office ATMs will have an English-language user option. Elsewhere, especially in more rural areas, it's difficult to find suitable ATMs. PIN numbers in Japan are comprised of four digits. In Japanese, an ATM is commonly referred to by its English acronym, while PIN is ansho bango.
MasterCard and Visa are the most widely accepted credit cards in Japan. Many vendors don't accept American Express.