You can eat quite decently at a restaurant for ¥2000 (about £13) per day total for lunch and dinner per person. A bar depends on where, and range from moderate to insane.
Souvenirs can be a few pounds each to the sky is the limit. The other major factor which you didn't mention is transport, not to mention where you will go. If within the same city, if you travel a lot, you can get a day pass in many places. Or get a 3 day Tokyo subway pass for 1500 yen.
http://www.tokyometro.jp/en/ticket/value/travel/index.html#anc03
The cheapest way into Tokyo from Narita Airport is the ¥1000 bus.
http://accessnarita.jp/en/home/
If you want to travel by rail across Japan, look at getting a JR Pass - you can save a ton.
See: http://www.japanrailpass.net
Even if you are staying in the same area, there are regional passes you can take advantage of as well.
http://www.japan-guide.com/e/e2357.html
You don't have to worry about running out of money so much though. If you have an ATM card you can easily get cash from any 7-11 or post office ATM, as well as some others, at a good exchange rate.
See:
http://www.jp-bank.japanpost.jp/en/ias/en_ias_index.html
http://www.sevenbank.co.jp/intlcard/card2.html
Browse the sites below on what to see and do.