how much it cost taking a taxi from Beijing airport to Tianjin city ?

Generally speaking, the cab fare is CNY 500 to 600 for the single trip.
If it's too expensive for you, you can also take direct long-distance bus at the airport to reach directly. The bus schedules are from 07:30 to 23:00 with the interval of around 40 mins.
It takes around 2.5 hours on the way. And the ticket fare is CNY 82 per person.
You can choose the way you like.