Japan is expensive- bring enough money!
The cheapest accomodation you will likely find will be about \4000 pp as there are not many youth hostels and when there are they are not often close, convenient or friendly!
Eating out at cheap restaurants such as noodle houses or okonomiyaki is usually as cheap as trying to buy food to cook. The rice, vegetables and fruit don't get any more expensive than in Japan.
It is well worth the trip but just be aware that if you run out of money you can't do it cheap or for free.
Also: ATM machines are not 24 hours and most only accept Japanese cards. This includes ones that have visa or mastercard stickers on them.
As with any country don't get caught out with the departure tax. In Japan as with everything else the price is especially high at a nice 2650 for adults.