The tuition fee is not expensive as far as I am concerned. It's $100,000 Hong Kong Dollar per year if not mistaken.
Accommodation (perhaps catering as well) in the university is relatively cheap, if I am not mistaken.
But the rest should follow the normal rate (ie expensive) in HK. Transport, entertainment etc.

But, I would say you should consider if you want to study in HKU.
One of the concern is you are not going to have a very good studying experience.
The culture is similar to most Asia places, still people that don't queue up for buses, still in buses can quarrel, simply throw rubbishes etc.
UK, US, Aus and other popular countries for studies not only do well in education part, but experience part as well.
That's why you don't listen people heading HK for studies.