On package vacations, single travelers are often hit with a "single supplement" cost added to the base price. To avoid it, you can agree to room with other single travelers or find a compatible roommate before you go from one of the many roommate-locator agencies.
Uniworld, 16000 Ventura Blvd., Encino, CA 91436 (tel. 800/733-7820 or 818/382-7820), specializes in single tours for the mature person. It arranges for you to share an accommodation with another single person or gets you a low-priced single supplement. Uniworld specializes in travel to certain districts of England, France, Spain, Italy, and Scandinavia.
For more general information, check out Eleanor Berman's latest edition of Traveling Solo: Advice and Ideas for More Than 250 Great Vacations (Globe Pequot), a guide with advice on traveling alone, either solo or as part of a group tour.