Sanya to Haikou by Bullet Train

Dear Sir / Madam,
This is my first trip to China. I am travelling to Sanya on 27th to 31 Dec. I want to try to take the Bullet ride. It is easy to buy ticket for daytrip to HaiKou and back to Sanya during this period?

What is the cost ( RMB) per adult ticket? Can I buy ticket on the same day of my trip? What is the travelling time from Sanya to Haikou?

Thank you in advance for your advice.

Best Regards,
Mr Wang Yong Ching
Singapore