Having tried to boom a cruie for two elderly relatives who both want a single room I know that it can be a problem. Most ships charge quite a lot extra for single accomodation. However I have found that QE2 actually has some single cabins. For a twelve night Med cruise in August the price for a single cabin is £1557 (plus you get $100 spending money on board).This is about £300 more than the price per person in a twin cabin. I booked through ######## who were incredibly helpful in checking the prices of numerous cruises for me.
Single Supplements vary from 25% of cruise fare to 95%. If you can be flexible with dates, try flocking through Teletext, you can sometimes come up with a deal with no single supplement. Many cruise ships operate a scheme whereby they will pair you up with another single person to share. Haven't tried this myself, but have friends who have taken advantage of this offer, mostly they have been paired with someone they got along with. As I usually cruise alone, I have found the Marco Polo (Orient Lines) to be about the best. Lovely friendly ship. They do have 2 single cabins which they don't advertise, they ate numbers 101 And 102, I have been in them myself and although compact you are only sleeping, washing and changing in them. Hope this has been helpful.
SAGA don't normaly charge single supplement but cruises can be expensive to start. Look for special offers. Give them a call. We found the staff excellent.
You'd better shop around as it depends on the agency. It can range from 1 1/2 to double the price of what it would be for a person sharing with anothe.