First of all, because the exchange rate of the Hong Kong dollar has fallen, this exchange rate difference is equivalent to a big discount. At present, the exchange rate between RMB and Hong Kong dollar is about 100:82. This is equivalent to a discount of 12% for all products. Moreover, the prices of some cosmetics counters in Hong Kong will be lower than those in the Mainland, which really means "the more you buy, the more you save".
As mentioned above, if you buy cosmetics in Hong Kong, some cosmetics will be cheaper than those in the Mainland. Not only that, there are many package combinations, which are more affordable to buy.
There are many kinds of drugs commonly used in Hong Kong families. Sometimes when I travel to Hong Kong, my relatives and friends at home will ask me to bring back some spare medicine from Hong Kong, because some functions of medicine in Hong Kong are different from those in the Mainland. For example, the Hong Kong version of Double is different from the mainland version. Hong Kong's Shuang Ren Fei can be taken orally, while mainland's Shuang Ren Fei is only suitable for external use.
The biggest difference between the Hong Kong version of milk powder and the milk powder in that place is the different formula, even if it is the same brand of milk powder. Hong Kong version of milk powder is strictly implemented in accordance with EU standards, and the quality is more guaranteed.
If you want to buy general goods with high cost performance, you'd better buy them in non-commercial tourist areas. Those stores are generally aimed at local people, so they are all genuine.