Just check the classifieds if you don't want to pay full price ...
Don't forget again caveat emptor, that there are plenty of second hand shops for mobiles as well.
cheapest prices are on possibly on this classified site and the hk expat site, most likely cheaper than sin tat or ssp, plus less likely chance of dodgy componets (apparently) replacing proper parts of phones.
if you travel between cwb and taikoo, then drop out at north point mtr and go to the shops in kam hong street (50-80m from mtr exit). 5-6 good shops there.