There are a number of Caribbean and Alaska cruises that have single supplements that range from 25% to 45%. These cruises are affordable with inside cabins available for around $600 to $1100.
There are a number of other cruises that, even with a single supplement, will still cost an arm and perhaps two legs. These are longer European, Asia and World cruises.
Holland America is the line that has the most discounts but all cruise lines have them on select sailings.
My advice is to get in touch with a travel agent who is knowledgeable about single supplements and have them do the looking for you.
Take care,
Mike