[Rauisufirippu]

Kinda same thing here.
Me and my friend decided to travel troughout Japan next summer too..

The avarage costs vary from 2000-4000 for about 2-3 weeks?
Depends on how much you spend on what though.
Read other travel advice threads around here.
Also wikitravel and check the thread I made in here, a lot of people gave me some good advice too.
Just read, read and read. You'll learn alot and you'll get quite some good ideas.

But also think about, what are your interests in Japan?

Cars? Anime? Culture? The sphere of busy cities? Food?