When I last looked at Royal Caribbean for single cruising there was a 225% supplement and even the 2nd flight had to be paid for!! It was actually cheaper to take a friend and not charge them anything to come along!
It is all based on supply/demand. There is no set figure but usually the mark up is 150-175%. You will just have to shop around and look for the bargain. The Celebrity Mercury is doing a 7n Alaska cruise on 6 July for only £833 for a single person which I think is astoundin.
The single supplement does vary from cruise line and even from each cruise. Queen Elizabeth 2 and Fred Olsen offer single cabins so the prices are usually better than if you paid a single supplement on other cruise lines.
P.S. just a note that the Celebrity Cruise mentioned above appears to be cruise only.
Mr Smith - of course it is cruise only for that price!!! Actually further investigation gives you a single cabin with flts on the 08Jun for a mere £1575.75. Bargain! If we know what Mr Atkins wanted we could advise...
Why not look at P and O cruises. In the supplement section under the cost, there is a percentage on how much single suppelment is. We are flying to Mexico then cruising the Panama. Two of us sharing are paying £1344. Mom is paying £1900 for the privilage of a single cabin. She is happy with this price, as alot of cruise companys want at least double the cost.
P+O offer several cruises with single supplements of 50% or less. I have just booked two for late Novemver 2008 where the supplement is 30% and 35% respectively - both are cruise only from southampton.