You're roughly looking at an average of 7,000 yen a day, including Accomodation. Thats an average though and depends on where you are going to be.

You will be looking about 2,000 to 2,500 yen per night for Accomodation in a Youth Hostel or similar cheap accomodation. (The hostels here are actually really good)

Id say (if possible) work towards about saving and having about 10,000 yen per day available and you will be fine in terms of Meals, Accomodation, Travel and extras (such as snacks etc) with maybe a little left over. If you plan on buying stuff here then obviously you will need to set your budget higher.

Again the cost reflects on what you would like to get out of the trip. If you want to be staying in Capsule hotels, traditional Ryokans, eating in fine resturants etc then the cost will be much higher. I would stick to Hostels mainly, but stay in a Capsule Hotel, Traditional Ryokan etc at least once in your journey to get the experience of the culture as well.

When I travelled we kept Breakfast and Lunch cheap and cheerful and then would spend more on the Dinners etc