The cheapest shoule be hostel offer bed and you have to share room with other, just like a youth hostel. That costs 65HKD per night. Or do you want your own room. Well, I recomment an hostel in the 7th floor of Ping On Mansion in Nanthan Road (below 200HKD per night), Yau Ma Tei. Mirador Mansion in Tsim Sha Tsui also have many hostel inside. Airport bus A21 just stop in front of the entrance of the building. You can pick one there. There is a homestay hostel beside Sogo, Causeway Bay, but I forget its name. :-(