i went to thailand for 13days and stayed primarily in phuket. there was a group of i think about 8 of us, and it cost around $2300-$2500 each with airfares, travelers insurance, accomodation, breakfast every day and all transfers.
It would depend on what you want to do, but if you are thinking of staying mostly in one place, you can book tours to different places (like around the islands or to sanctuarys up in the mountains) very easily when you are over there. it is much much cheaper to organise travel of this kind once you're in the country, there's little stalls all over the place that sell them and we managed to find one at a good price that was fantastic and not at all dodgy. We did a few different things, but whole day trips with all meals cost about $35.
I'm not sure how doing Thailand like this compares to doing a full tour staying in all different places though, which i guess is more what you are after