what is the cost taxi for six from shantou airport to dapu?

Do you refer to the Dabu County in Meizhou, Guangdong? It is about 200 kilometers from the airport. You may charter a van there since you have six people. The fare may be around CNY400-500 for a single trip.
In addition, you can also take the airport shuttle bus to Shantou City Termianl. Upon arrival, you just need to walk about 500 meters to Shantou Central Bus Station. Then you can take a direct coach to Dabu County.