For around 3k you can get some small studio flats in the Shek Tong Tsui and surrounding areas, if they are available instead of renting a room. If you don't mind living on Kowloon side or in the new territories, that would be enough too.
HKD5100/month for that size is quite good, depending on the condition of the place. But I would like to point out that a trip to CWB is more likely going take around 40mins-1hour. It actually is faster to go to Mong Kok than CWB from that area. LOL