Yes, market penetration of 141 would be another way of getting to the same thing...
My casual estimate -- based on a fair understanding of the mongering scene in major cities back in the US -- was that 1 in 1000 women from age 18 to 39 was somehow involved in the pay-for-play scene. I might have been off by a factor of two, but I doubt I was off by a factor of five.
I have to believe that this ratio would only go up with greater income inequity. And HK -- while not exactly as bad as China in this respect -- does have considerable inequity of income and wealth. So I would have to believe that the ratio is > 1 in 1000.
A quick check of the HK demographics reveals that there are 1.3 million women in HK between 18 and 39. So 4000 WGs would be about 3 per 1000. If I had to place odds, I would bet on the "over", but I'll bet you're in the ball park.
But then there are the Wan Chai filipinas and Sunday maids and other assorted "grey zone" types... which can only raise the number...
All in all, HK is not a bad place to be...
Have fun, CGP