It is quite expensive in Paris. Which arrondisement do you want to stay in?
This is not a bad site to try http://en.parisinfo.com/paris-hotels/other-pla ces-to-stay-1/young-paris/
Don't, as Beunchico says :), let the word hostel put you off
From what I have read here and on other travel sites, hostels are generally cheap and clean. I don't know though, whether a hostel can take you long term?