What could be the cost for a taxi from the Qingdao Airport to Weifang downtown?

I'm afraid that's not reasonable. As I know, it may cost at least CNY300 to Weifang.
In addition, there is a shuttle bus at the airport to Weifang directly which takes around 2 hours. The ticket fare is CNY80/person. The buses depart at 10:00, 11:30, 13:00, 14:30, 15:30, 16:40, 18:00, 19:30, 20:50 and 22:00 respectively.

Thanks for your reply. I considered a rate of 3,85 per km. as above indicated, for a distance of approx. 140 km. - I'm organizing the journey of one of our technician in Weifang and looking for the best solution from airport to city.
300 CNY for the taxi it's fine. He arrives from a 10 hours flight and we would like for him a quite comfortable solution.