Yes, they sell it in packages, but I honestly don't remember what the cost was (for the Mariner which I assume would be the same). I want to say it worked out to be $40 for 15 minutes as the smallest package and the price per minute went down for the bigger ones. Maybe Sonny can answer, but I know he uses his C&A coupons for the internet usage.
I do know it was much cheaper than using the ship to shore phone if you are intending to use it to check in back home.
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, Magic 1/14
NCL Dream October 2006, RCCL Mariner OTS June 2007, Celebrity Silhouette 2013