U can always negotiate the commission rate to the agent before you sign the preliminary S&. Since the agent is collecting commission from the buyer and seller, and the rate was stated on the Pre S&. U can tell how much both the parties are paying.
For the house that i biught recently, the agent is getting 0.7% from the seller, then only i start negotiating for lower commission, and i only manage to get 0.9%.
I have a friend (buyer) who only paid HKD500 the last time he bought a house.
I am a first time house buyer in HK, thus only aware of this after i bought the house.