"Nice" for dinner has the biggest variance. To be safe, I would say allow 5,000 Yen daily on average per person.

Bakery items and a coffee at a cafe should cost less than 500 Yen. A simple lunch can be 500 to 1,500 Yen. The rest goes to dinner.

Note that in Japan, you don't walk around eating on the streets. If you buy takeout, you will need to find a park bench or go back to your hotel room to consume your goodies. Since Japanese fast food (rice bowls with toppings, ramen, etc) don't cost a whole lot, you might find it worthwhile to sit down for just 30 minutes, take a break and eat a decent meal.