Plenty of places have beers from $25 up during happy hour (up to 9pm generally) after that, you're looking at $45 up. If you want the posey bars and clubs then $65 up is normal, but places like the Hong Kong Brew House, Bulldogs, etc. have beer under $50 a pint all night. Plenty of hard-up types buy their beer in 7-11, particularly on weekends where many people spill out of the bars into the street anyway so you don't actually need to go into any bars...
Asian girls look younger than western ones. Be careful about prejudging who are pros and who are amateurs (and who are in a sort of grey area in between). My guess is that the average age of Wanchai pros is about 26-27. Wanchai is a complete mix of "normal" bars and brothels masquerading as bars. Brothels come in two basic forms: the "curtain bars" with people outside encouraging you to come in for "just one beer" which have a fairly fixed staff on the payroll, and bars which house freelance girls working on commission only (this includes places like Fenwick, Neptune, etc.). It's reasonably obvious which type is which.