Rental will vary greatly depending on location, size and services, etc
For your consultancy your choices would include the usual: standalone commercial property, serviced office, shared office (refer to classifieds), virtual office and residential.
For rough guide on costs refer to Virtual Office HK, JumpStart, PrimeCenter, Regus and many others...
"Virtual offices" provide you with a registered address, receptionist, meeting rooms, etc - but you can base yourself somewhere else entirely (if you require that "professional image" at a reduced cost). Again costs vary but typically less than HKD $500/pa + meeting room cost only when required.
In HK, there are also many other places to locate your business that may seem less glamorous and unconventional, but I would highly recommend to consider them also for cost considerations (ie. I've run businesses out of storage units, and old warehouses / factories).
And since I get "boo-ed" on these forums for being a stickler for the law, I might as well add, that there are many HK businesses (especially restaurants + retail) that are indeed run out of residential premises due to the high cost of rentals (& licensing). You could combine that idea with a virtual office which many have done where customer on-site visits are rarely, if ever, required (IT consultancy, tech startup, web designers, etc).
I would always recommend incorporating as a Ltd Co (rather than a sole proprietorship) to limit your liabilities and only look for an offshore entity once business is stable. For a consultancy you should take out Professional indemnity insurance. With an office, you need Public Liability insurance as well. (Not sure of your business, but check out licensing requirements)
I think Football's resources are excellent starting points to get you going...