In 2007 I spent a little more than a month in Thailand. The main purpose with the trip was to have LASIK eye surgery to meet the medical requirements to obtain my commercial pilot license. The surgery itself did not take more than 20 minutes, and after a week of recovery in Bangkok, there was plenty of time to explore the rest of the country. I traveled with my good friend Lasse, and we visited Phuket and Hua Hin together, and had a great time, but the last 2 weeks I was on my own.
I had spent most of my travel budget when I was traveling with Lasse, so the last two weeks, hotel rooms and lobster dinners had to give way for hostels and street food.
One of the most amazing things about Thailand is that you can live comfortably for almost no money. During the two weeks I traveled on my own, I did not spend more than $700 including transportation back and forth to Bangkok, excursions and food.
Thailand is a little unusual in that there is a continuous trade-off between all-consuming mass tourism and all which that implies in terms of prostitution and Happy Viking Scandinavian Bars on the one hand and the not-so-exploited but also considerably less comfortable backpacker havens on the other hand.
Koh Phagnan is still more backpacking than boom-boom-ing, and it is a really good place to go for solo travelers. If you want company, you can just head down to the beach and chat with other backpackers, and if you want to be on your own, you can still go to the beach and just quit the chatting:)
Haad Rin is (in)famous for Full Moon Parties, but when I was there it was quiet and calm and really cheap. I stayed at a very nice hostel, Same Same, which is Danish-owned and in walking distance to Leela beach. I had a private room, and back then, there was no hot water, but I can see on their website, that hot showers are now available. When I was at a high school reunion party last year, I was talking to an old classmate about Thailand and traveling, and guess what? Two of her friends now owns and run Same Same. What a coincidence!