Best to compare TO and DIY - its not always cheaper to DIY. We usually go DIY but tend to use hotels not in the TO brochures when visiting Thailand, so difficult to compare prices. Our last Thai holiday cost us about £680 pp for 15 nights 3 centre (Phuket, Krabi, Bangkok) with internal flights and private transfers throughout, staying in a mix of three and four star properties - hotels and serviced apartments.

I've copied your query to the Far East forum as you may also get some feedback there.