How much does it cost from Xi'an to Fuzhou if by train, or by airplane?

There is only one train between the two places: 1218/1215.
It takes about thirty hours on the way. The ticket fare is 337 to 362RMB for the hard sleep.
For the flight ticket, it is hard to tell the price, because you may get a big discount if you book the ticket in advance. As far as I know, Tianjin Airline, Xiamen Airline, Hainan Airline and Shenzhen Airline have flights to reah FZ and you may find the price on their official websites.