It depends on the cruise line. If it's a single cruiser, the line can charge 150% to 200% of the double occupancy rate. Some lines have small rooms for single cruisers, and the rate may be better than the upcharge of a standard stateroom. Here
is a link to another discussion regarding the same topic. It's a few years old, but I think the general rule still applies that rates for singles are 150-200% of the double occupancy rate for singles.
2007 - Majesty of the Seas
2009 - Enchantment of the Seas
2010 - Freedom of the Seas