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It's impossible to generalise as good offers come and go. It is perhaps better to choose a cruise line which has dedicated single cabins rather than doubles for single occupancy eg Saga, Fred Olsen. It best to shop around.
I disagree with Miller, I have travelled as a Single Occupant with P&O six times now. There have been varying supplements from 0% to 60%.
It also really depends where you want to cruise to, and at what time of year you want to go. Examples I have experienced are; June Fjords 7 nights 60% supplement, November Canaries 0% to 35 %, November Eastern Med 35%.
I have found that the earlier you book as a single, the lower the single supplement is. Sure there will always be some great offers if you book late, but in many cases you don't get great cabin locations.
My advice is to decide where and when you want to go, and then contact a cruise agency, they will be able to give you quotes from various cruise lines.