The true dogsledding tour is very expensive ($300 a person or more) and anything involving an aircraft will be $400 a person.
The whale watching tours are in the $80-$100 range and the railway tour is cheaper. Its hard to put an average on the excursions because they very so much in price depending on the excursion. As far as booking them in advance, I would book any tour that you MUST do now. You don't have to book through the cruise line to do so early either. Go to the internet and search on the port you are interested and the tour and you will find local operators that will reserve you a spot now.
There are always local tour companies that offer the same excursions without the cost of the middle man (the cruiseline) BUt if you have to do something, book online or through the cruise line now. Or you might not find a tour with space.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007