Originally Posted by corporate-wage-slave

No, it's not a fixed rate and will vary by traffic conditions. In normal circumstances you would be looking at HK$400-500. However unless you arrange a van or the premium Uber options, the usual HK taxi is quite small, and even 3 people is a tight fit. Formally though HK taxis are licensed to 4 or 5 passengers, with toddlers not counting. So depending on ages, you are probably going to be in 2 taxis unless you dig out alternatives. You can expect a driver to be fairly strict (e.g. asking the age of children).

The Airport Express has a group fare for 4 passengers which works out at HK$70 each person, and a 2 + 3 person ticket, which comes in at a fractionally more expensive HK$400. So if you are in 2 taxis, cost and time wise, the Airport Express is likely to be best, for convenience 2 taxis or a prebooked ride is going to be an alternative. If it is your first visit to HK then the train ride is quite a good introduction to the look and feel of the place, given the views and people watching.