I've travelled from 12th october until 31th, with my girlfriend, and we had sleeped in hostel only in Perugia one time, and NEVER sleeped outside!!!
Better try to check HospitalityClub, it's for free and more fun then hosteling!
But if you decide to hostel, you can check prices in this website under countries section. Or rail.shop/hostelworld
. Averages in western/southern europe is about up to 20-30eur/night in countryside and 30-40eur/night in bigger cities. I'd better choose countryside with a railway, you often can find it!
. For eastern Europe - don't know, max 20eur/night, maybe you'll find even 5eur/night in Poland or so on...
And without camping equipment you'd have 2x less backpacks!
you can take more clothes, or have easy backpacks for beginning and can collect things you boy on the way, even special food or so on.. But if you plan to use hospitalityclub, take sleepbags - you'll be more vulnerable!
Have a nice travel!