NosDa@The Riverbank is sort of mid-way between being a hotel and a hostel. You could book 2 twin rooms,with en suite facilities, for �26 per person per night. (i.e. �104 per night). Or, you could book a 4-bedded mixed dorm, also with en suite facilities for �18.50 per person per night (i.e. �74 per night). Continental breakfast is included:
http://www.nosda.co.uk/
The Nomad Hostel is cheaper and also offers a choice of 2 twin-bedded rooms or a 4 bed dormitory. However, bathroom facilities are shared. Typical prices are �15 to �17.50 per person per night in twin bedded rooms (i.e. a total of �60 to �70 per night) or �14 to �16.50 per person per night in a dormitory. (i.e. �56 to �66 per night in total). Breakfast is included.
I can't find an independent website for the Nomad Hostel but you can find it (together with NosDa and a few others) at
http://www.hostelworld.com
(where you'll also be able to read reviews of the hostels).
Chris