I have no idea what Celebrity charges to do the WhitePass Railway from the ship or what they charge for booking it on your own.
I have done the WhitePass Railway and the views are well worth the trip. It is a small walk into town if you want to catch the train there. People I have talked to found that it was worth booking from the ships instead of on their own.
When I took the train I booked it from the ship and found it was well worth the price but I was not on a Celebrity ship.