When I'm in HK, I always stay at Dragon Hostel, Room 707, 7/F Sincere House, 83 Argyle Street, Mong Kok, Kowloon, Hong Kong. It's HK$210 a night or HK$240 if you want en-suite. It's clean, a good deal less ghetto than Chungking Mansions, and best of all no persistant tailor, hashish and fake Rolex touts everywhere. http://www.dragonhostel.com/
I find the TST part of Nathan Road to be such a hateful place and is strictly for unwitting tourists, much prefer Mong Kok. This is real 'Hong Kong' IMO.