The only way to go to Sanya directly from Hong Kong is to take flight. For other ways, you can take long distance bus at Futian Bus Station in Shenzhen, which means you also need to transfer at Shenzhen. The bus to Sanya leaves at 15:00 daily. I recommend Yonghe International Youth Hostel to you, it is located about 2 kilometers to the place of Miss World competition, and this hostel costs around CNY30-60 per night. From Sanya to Hong Kong, there is also no direct bus or train, the best way is to take bus to Shenzhen and then go to Hong kong.