I was in Hong Kong in very early July. The main carrier, PEOPLE's (China Mobile's Hong Kong branch) didn't have the 3GS yet. They were still selling 3Gs. I didn't really see anywhere besides an actual retail store for the network in which you could buy an official iPhone. I am pretty sure the prices were comparable to contract prices here in the US, if not more. I'm sure the non-contract iPhones (if you can find them) are pretty pricey. As jav6454 said though, be very careful, there are a LOT of knockoffs in Hong Kong (even though it's illegal to sell them)
PS - I'm not sure if Vodafone sells the iPhone or not (I think they do)... but if you are set on buying one, you might wanna check them out (They're the European carrier (i.e. more reliable)).