Place: Popular HK mongering location
Rooms: Upper floors  
Rent: ~30,000 HKD per month (less)

Info source: Anecdote with girl     


Apparently, business is tough and there's high turnover for the working girls. In another words, many girls can't clear $30,000 HKD a month.   

At $600 per client, a worker needs to clear at least 50 clients for break even. Or, about 11 clients per week. Or 1.5 per day.      

I will use the HK GDP per capita to determine if the job is worth it (or otherwise find any other job). HK GDP per capita in 2018 is $48,600 USD or approximately $370,000 HKD according to the World Bank. I used GDP per capita as I expect demand for mongering will increase or follow the productivity of the economy. Especially when the location is close to the financial hub.   


So alas, for this work to be worth it, a seasoned working girl needs to clear 3.3 clients per day. If she does so, they'll take home $1084 HKD a day or $396,000. In a month, they'd need to clear 105 clients.      


My sensitivity goes up to 150 clients/month. Even then, the girl only clear $1972 HKD a day.

Your rental figures are way off. The average rental for a WG’s room, at least in HK or KLN should be about 9 to 12K HKD a month.