About Shekou ferry terminal and the nearby transportation.

It is about 80 kilometers between the Shekou Port and Changping, so it will be about 400 RMB by running on meters with a taxi. Although you can negotiate with the driver for a reasonble price, I do not think it will be so cheap.
In my opinion, You can take the Bus Airport Line 10 to reach Nantou Bus Stop (the tenth stop) from Shekou Ferry Terminal and walk to the Tianqiao Branch of Nanshan Bus Station. There you can take the long distance bus station for Changping.
The distance between the Shekou Port and the bus station is no more than 10 kilometers, so you can also consider to take the taxi to reach there. The taxi fee is about 40 RMB running on meters. You had better ask the driver to run on meters instead of the negotiated rates.