I did some searching, and there still are some public government English-medium schools. I also found this quote for Hong Kong International School:

Finances: In the 2007-2008 school year, about 94% of the School’s income derives from tuition and fees. Annual tuition rates are as follows: primary-grade 5: $16,142; grades 6-8: $16,761; grades 9-11: $18,644; and grade 12: $18,708. There is an application fee of $1,500. In addition, the School requires payment of annual capital fees or debenture. (All fees are quoted in U.S. dollars.)

So your minimum costs would be around US$100,000 per year for this private school.

I also found this in an article:

Comparing the cost of renting an unfurnished three bedroom apartment, ECA’s data shows that Hong Kong is by far the most expensive city, with an apartment in a popular expatriate area costing approximately US$8,592 per month on average," said Lee Quane, General Manager of ECA in Hong Kong. “That's 17 per cent more expensive than the average rental price in Tokyo, which ranked second in the survey, and 150 per cent more expensive than 15th placed Singapore."