A decent flat (IMHO) for a family of four will cost about 40-50k per month unless you want to make your family suffer and end up divorced.
Not that much IMO. In Hong Kong the sky's the limit, but I would say a decent, modern, fairly well-located 3-bed apartment could be had for less than 30k per month.
Obviously you're not going to get a house with 1000s of square feet on that salary, but it's Hong Kong.
In reality 80,000HKD is a very good salary in HK, and is far above the average local salary. As a point of reference... local shop workers/ waiters earn around 40HKD per hour (8000 for a 200 hour month), most new graduate jobs offer 9.5 to 12k per month. Mid-level managers earn around 20 to 25k per month.
Even for expats, the number earning over 80k will be a very small percentage of the overall total. And in no industry (except for very senior executives) does anyone offer generous expat benefits in HK anymore.