I did that many years ago. In terms of cost, relatively speaking it is as cheap as you can to own your motorised transport in HK.
However, the real killer is parking. Hong Kong has a very don't-give-a-monkey-about-motocycle attitude. There are FREE parking in older districts, which are nearly always fully occupied. And they are fast disappearing. But for newer places, there are no parking space whatsoever. You need to pay for it. As far as I recall, only very very small number of places have dedicated parking spaces for motorcycle. That is, you pay full whack for a parking space to park your motorcycle. And when you go around places, how do you fancy paying the full hourly rate to park your bike in a car park? It used to be OK to park on pavement etc. But that depends on your local police's attitude about it. In some disctricts, motorcycle are being seen as easy target for tickets. I used to go to work on motorcycle. My office was in the Central and parking wasn't a problem. When my office moved to Quarry Bay, I left my bike on the pavement and I got 3 tickets in 4 weeks. In the end I just gave up.
One word of warning too. Speed camera is WIDELY used in HK, normally in the most "sensible" places. That is, any stretch of straight road with a LOW speed limit. They are mobile units and as a rule of thumb, well hiden without any warning. If you own a powerful bike, any "spirited" drive will soon lose your license for sure.
About 10 years ago, there were a very lively expat motorcycle scene on Sunday morning. They normally went and have a spin in Ting Kok Road and have coffee in a little Store in Luk Keng. Even saw a Duke 916 once. But I think most of that communities have left HK. You may try nevertheless. Take KCR to Fanling and there is a frequent minibus service to Luk Keng. Worth the trip for the scenery even though you may not meet any motorcyclist.