To be more accurate I will need some deographic data like the how many inhabitants HK have , what percent speak english , how many foreign peoples spend vacations there average ...etc.
In my humble oppinion and maybe missing some important key information I think
Airport.hk might be ranged between 100.000 - and 1.000.000 USD but this depending maybe by the firms involved in managing the HK airport to invest in this domain .
Mortgages.hk under 100.000 as it is adresed to english speakers who most probably are just visiting HK and not take there their mortgages .
Single.hk between 10.000 - 100.000 USD again depending on the local investor desire to buy/develop the domain .
Holidays.hk between 10.000 - 100.000 USD
Those are just my personal feelings about your domains posible value . Other more experienced guys might think different . Good luck with your sellings .