The last time I stayed in Japan, I spent between $4000 - $7000 a month. There wasn't much unusual about my stay, I used the shinkansen a lot, went out dining with friends and did some shopping. It really depends on what you are doing, stay with relatives or in a top hotel at Shibuya station, buy fancy devices you actually don't need, try the local top grill restaurant (6'ooo Yen for 5 slices of meat) etc. etc. You could as well have a complete menu for as little as 500 Yen and stay with the monks in a temple for a few bucks.
If you go there as a photographer, you won't need much. A cheap place where you can sleep, the railway pass and food is cheap anyway. Keep some money aside for unexpected incidents, i.e. some places might be full (on weekends, public holiday) and only expensive hotels have free rooms. If you intend to go there on holiday, better keep saving up more money and go when you can afford a normal trip. How much fun would it be to fly over to NY, but you have almost no money to spend...? It's ok if you visit friends.
Japan is not fun without money if you go as a tourist. I don't like to say this, but the more you can spend, the merrier ;-)
Japan is an expensive country, similar to Switzerland. And in the end you always wish you had a bit more left to spend. I always come back with absolutely no money left in my pockets.