I would allow for...
2 tickets for Narita Express to Tokyo station = 6000 (roughly)
Taxi from Tokyo station to Ginza area = 3000 (comfortably)
Dinner for 2 = from 2000 to 6000 (of course you can spend more....)
Coffees etc. = 1000
Subway passes = 2000
So you can manage comfortably for the first day or so with 18,000yen (roughly $160)
However, I would be inclined to carry twice that. Just in case....
(I always take at least 30,000 from the ATM each time).