Thanks again guys. However, even though I realize that I don't have any real knowledge about accommodation in HK, I also see that some of you might be overtly pessimistic. Just as drumbrake adviced, a place called the Nest is apparently offering service apartments in Mong Kok. I've seen some other places as well, such as Yesinn, City Loft and Studiostudio. All these places offers studios for approx 8k (even less in some cases). I don't know the quality of these places, but they look nice at pictures at least. Anybody else that can recommend serviced apartments?
And also, I didn't get any answer on my other question regarding whether it is possible to rent a normal apartment for 5 months - that is, not a serviced apartment?