I just came back from Hong Kong on September 20th. Recently, Lancome and Estee Lauder in the DFS of Hong Kong Airport have put up advertisements for "the lowest price in Asia". I bought the Lancome small black bottle 100ml in Pudong, and it only costs 989 in Hong Kong.
I didn't buy SK-II, but I suggest you make a reservation on Beijing Day first and check the price in Hong Kong when you come back. If the price in Hong Kong is lower, you can cancel your reservation separately when you return to Beijing.