Quick answer: Tokyo runs about EUR 120 per day mid-range - noticeably less if you eat local and walk. Cheapest window: March–April (cherry blossom) or October–November (autumn leaves).
What things actually cost
bowl of ramen
8 EUR
metro ride
1.30 EUR
konbini onigiri
1 EUR
coffee at a kissaten
3.50 EUR
temple or garden entry
3 EUR
izakaya dinner with beer
22 EUR
Eat well for little
Ramen at a ticket-machine shop - order via vending machine, a full bowl runs 7-9 EUR, Ichiran or any busy local spot works
Sushi at Tsukiji Outer Market - fresh nigiri sets from about 15 EUR, go before 10am to beat the queues
Conveyor-belt sushi (kaitenzushi) - plates from 1 EUR each, chains like Sushiro or Kura are cheap and reliable
Konbini food - 7-Eleven onigiri around 1 EUR and egg sando about 2 EUR, genuinely good and a real local habit
Izakaya dinner - small plates plus beer in Ebisu or Shinjuku, expect 20-25 EUR per person
Money mistakes to avoid
Planning too many neighborhoods per day - Tokyo is huge, cluster sights by area or you lose hours on trains
Not getting a Suica or Pasmo IC card on arrival - buying single tickets for every ride wastes time and money
Trying to tip - it is not done in Japan and can cause awkward refusals, just pay the bill as printed
Buying a JR Pass for a Tokyo-only trip - it only pays off with long-distance shinkansen travel, use the IC card instead
Transport without the tourist tax
From Narita take the Skyliner (about 17 EUR, 45 min) or from Haneda the monorail (about 4 EUR, 20 min) into the city. Get a Suica card and use the metro and JR lines - most rides cost 1.10-2 EUR and trains stop around midnight.