Cost for trip to Japan?

me and a friend are going to Japan for a 3-4 week stay around may/june.
I've been to Japan before, but just with my parents.
I have family for both of us to stay with, so hotels may not be a problem.
I was wondering how much it's gonna be for the airplane tickets, food everyday, rail tickets/other transportation/and entertainment. Thanks!

This is a pretty open ended question and lacks a lot of details that would be needed for an answer.

A few questions I have for you are:

Where are you coming from? What do you plan on seeing in Japan? Where will your base be, i.e. where do your relatives live?

I'm travelling to Japan in November for 10 days. I'm visiting Osaka, Kyoto and Tokyo, and I have allocated 100,000 yen for my trip. This doesn't include shopping and souvenirs, but it covers all the activities we will be doing, food and trains. 15,000 yen of that is for the bullet train though, which can be expensive. I think 300,000 yen would be enough for 4 weeks perhaps, if you do not need to pay for hotels and if you don't do expensive activities every day.