Depends on the brand of Vodka you want. THere is a recent price listing in the discussion on the Funtime forums Carnival Funville Online Community | Cruise Blogs & Forums | Carnival Cruise Lines
From it it says Skyy is $50 and many are at $60. But a bottle of Grey Goose will set you back $80.
Rum is $50 or $60 depending on brand.
Be careful with buying Bon Voyage liquor in the Texas ports. They can't legally give it to you until they are in international waters. So at best you see it late at night but they tell you not until the morning after departure. So, if you doing a short cruise from Galveston, your time to consume a bottle is shortened by a day.
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