That will be a good budget - you can expect to live for well under that in eastern Europe, though in some parts of western Europe you may go a little over. In September, you probably won't see a huge price difference, between high and low season, except perhaps on the flight, which would help enormously. Accommodations can fluctuate by between 10 and about 30% in the off season, but it depends a lot on the exact location - some cities are popular for trade fairs and business destinations, and they sometimes up their fares during the winter, particularly during the weeks. Mostly this is typical in business hotels, which probably won't affect you, but it's worth knowing about in advance. Hostels don't always drop their rates, it just kinda depends. You can get an idea by checking hostel search engine on the boards, and the individual hostel websites if you need more info.