Hi there,
Thessaloniki Hostel Backpackers Refuge is a small hostel and rooms are on the smaller side but it is clean and homely.
Another option would be the
Hotel Avra which is 7 miles away from Thessaloniki. The hotel offers AC, TV, telephone and kitchen.
There are not many hostels in Thessaloniki for less than 10 EUR per night.
Hope this helps
VacationBuddy