Solo journeys in Thailand offer infinite opportunities to make friends and meet locals, and it's easy to change your plans without upsetting others. However, solo travelers are something of an oddity here. Thais love asking foreign visitors, "Are you married?" They themselves commonly marry quite young and start families early, so many assume single travelers to be sad or lonely. Be ready to be offered sympathy, if you're traveling alone, and don't take it personally. Show them photos of family and friends so they can see you're not alone in the world. Also be prepared to be pestered sometimes; Thais don't share Western notions of privacy, so they never think of "giving someone space."
As for expenses, some hotels offer discounts for singles. If, however, you like resorts, tours, or cruises, you're likely to get hit with paying a per-room rate regardless. Single travelers can avoid these supplements, of course, by agreeing to room with other single travelers on the trip. Another way to reduce costs is to find a compatible roommate before you go, from one of the many roommate locator agencies.