As per my knowledge, the agent will charge both the employer and the maid separately. The agent used to make lot more from new maid, HK$12,000 or more. That explains why the agent always sell new maid instead of local miad. The agency fee is very flexible and sometimes negotiable.
This huge agency fee in fact, is somehow illegal because there is some kind of restriction on the agency fee paid by the maid, 10% of the monthly salary.