I look at it this way--when they pick you up a tthe airport, it's at least an hour of their time from when they meet you as you come down that elevator until they get you to your hotel. Eight people is probably at least 10 bags, that's a lot of luggage that they need to handle.
When they take you from the hotel to the port plus a grocery stop--that's at least 2 1/2 hours of their time...plus again a lot of groceries and such.
On the way back it's again at least 2 hours of their time--they'll be waiting at the port for you, load the bags into the van, etc.
Especially from the port they can't combine the trip--so they're driving either out of back empty.
If there were only two of you, then yes, $20 each way might be fine. But you have a large party with a lot of luggage and two "deadhead" trips.
The shuttle driver at the airport gets $1-2 per bag--for driving you and fifteen of your new best friends ten minutes between the park and ride and the terminal. A cab driver generally gets 15-20% of the fare, PLUS $1 per bag. You're suggesting less than that...and the service you'll receive will be a LOT better!
Anne