Some cruise lines do allow single passengers to share BUT you never know who you might get, eek!. Or you can pay a single supplement which varies from line to line and might be less than 200%. For example a cruise I have booked for this September is about 38% and for September 2009 16%, hooray, at least until discounting kicks in for late bookers.
Don't know whch cruise line you plan to travel with but some ships have single cabins which are more pricey than if you travel as a couple but are pretty good value.
Every cruise line has its own policy on single occupancy supplements. Many charge double (less the cost of the second air fare), some charge £xxx per week supplement e.g Ocean Village, while P&O for example vary the percentage supplement across all their cruise offerings (see there brochure for details). Having said that there are often special deals available so its worth contacting a few cruise agents and asking what special offers they have available and keep asking on a regular basis as these offers come and go.