I don't know about the Liberty, but on the Majesty, it was about $2.00 including the 15% gratuity that is automatically added to the bill. If you think you will be drinking more than three sodas a day, then it is probably worth it to get the soda pass.
2007 - Majesty of the Seas (Bahamas)
2009 - Enchantment of the Seas (Western Caribbean)
2010 - Freedom of the Seas (Western Caribbean)
2013 - Golden Princess (Alaska, roundtrip from Seattle)