Hi. How much is the taxi fare from Xiamen Airport to King Century Hotel, Zsongshan?

Hi, it is too far, so the taxi fare shall be very high. Besides, it may be a little hard to take a taxi to reach. As for your case, here is my suggestion:
Take a taxi to the local Hubinnan Bus Station, where you can take the direct bus to Zhongshan. Two direct buses are on schedule departing at 19:00 and 21:00. Upon your arrival at Zhongshan, take a taxi to your hotel exactly then.

For the hotel, do you refer to the one in Zhongshan city?
If so, I think you need to go to Zhongshan city first and then take taxi to reach your hotel.
From XM, you can take bus 27 to reach its terminal at Hubin Middle Road. Then you can take a taxi to reach the Hubin Long Distance Bus Station to take long distance buses for Zhongshan. The long distance buses are at 19:00 and 21:00.
Upon reaching Zhongshan, you can take a taxi to reach your hotel directly.