From Xian to Hong Kong on 11/25/09, there is one direct flight and several connecting flights with transfers at Shanghai Pudong, Beijing, Taipei or Guangzhou available. Personally, you can choose to take the direct fight, which is cheaper and the transfer time is 2 hours which will be sufficient for you. Its detailed flight schedule is as bellow:
Xian- Hong Kong 1420/1700 MU203, China Eastern Airlines

For the detailed cost, I am not clear. And you can get it by consulting the airlines or some booking agency.