I booked a hotel in Hong Kong in Ctrip. After a few days, the price was reduced, and they didn't even refund the difference.

If Ctrip doesn't have this guarantee, it won't give you a refund. Hotels in Hong Kong are all like this. Sometimes you book 1 1,000 yuan in advance, and only sell it for a few hundred yuan in the next few days or the same day. Of course, if you book above 500 yuan in advance, it will rise above 1500 yuan on the same day. So the price reduction will not be refunded to you. If you book in advance, the price will increase and you won't be charged more. This is the market principle.