Most of the mainstrean lines have a full 100% additional singles supplement.
Holland America has one of the most liberal single policies. They will find you a roommate, and if there is a spare cabin available they will give it to you for no extra charge.
Other than that - the luxury lines have some sailings where the supplement is lower, as low as 25% additional.
With every cruise line it can vary by cruise. Another option is to go with a singles group.