Last time HK took on expats was over 20 years ago.
But based on previous experience they will likely offer salary at the top of ATCO II rank for 20 years experience, which is point 44B on the civil service pay scale.
Civil Service Bureau - Master Pay Scale
Possible gratuity would likely be 15% (not 25% as per 20 years ago) which just offsets the flat tax rate.
The $64,000 question will be accomodation allowance, if any.
HKG is now recognised as the most expensive city in the world in terms of living cost. Rental costs are exhorbitant unless you are prepared to live in a shoebox. (Correction; even if you live in a shoebox)
If you have kids of schooling age, forget it. As UNIFO says, debenture and term fees for ESF schooling is prohibitive.