How much would it cost for two passengers to go from the sea port to the Great Wall and back

Do you refer to Tianjin Port? If so, I’m afraid it’s hard to find a taxi from the port to the Great Wall as it's too far away between. I'm not clear about the fare either. For your reference, a sinlge ride from Tianjin port to Beijing downtown may cost CNY400 – 500 around.

If you really want to travel between the sea port and the Great Wall, try to drop a mail at bookingtravelchinaguide.com. I know they have car rental service. You may see if they can help you with your route.