Cab Fare will be roughly about 20 per person. And you will be in a van with everybody else. There are no individual cabs. It is all shared transport.
If you are not planning on getting a vehicle in Cruz Bay, I would suggest taking Westin up on their offer. As you will have 1) transport to the dock 2) Ferry cost 3) Transport to Westin.
Also, while is St John, do get a car and head to the SE point of the Island to the Rams Head. It is a nice hike to the top and watch the sunset. By get your butt back to the beach before it is dark. It is very safe, but the trails are not lit at all.