Yup you will have to stay in Youth Hostels with that budget and just about scraping by with that amount of cash, if 400-600 is your actual true budget then i think a month will be quite hard. On average for me when i stayed, we stayed in youth hostels and each night of accomodation in a youth hostel on average was 20 euros which is about 27 US Dollars. Did not include food/transport etc. This is the average and norm price for a youth hostel per night.

That said, any hostel i did stay over there were very good and had alot of amenities like internet, kitchen, showers etc (all shared/communal). Depending on where you stay but i recommend the K House hostel branches. There is one in Tokyo in the Asakusa district and found it (and all the others) to be a friendly, high quality and low cost hostels (again it was about 20 euros a night).

To keep costs down as much as possible for accomodation i recommend a) Obviously using Youth Hostels and b) choosing Dorms rather than private rooms. Dorms will be the cheapest form of room charge since there are so many beds. The only downside is that you may have to share with strangers but while we stayed we honestly didn't mind it at all (in fact we met interesting people during our travels).

If you plan on staying in the same area for the month i would try as much to book either the same place for the period well in advance, or do bookings in different hostels close enough to each other well in advance also. If you leave booking to the last minute you will lose out on getting accomodation booked.