bus from xian railway station to local airport

Hello,
I will be arriving at the railway station at 7.50 am coming from zhangye. I have a flight to Shanghai at 11am. Are there shuttle buses running from the station, and if so when and how long do they take. I know this is rush hour so it may take longer.
Also in case there are no buses leaving close to when i arrive, how much should i pay for a taxi so i don't get ripped off?
Thanks