Hi
In September you should be able to grab a real bargain. I can highly recommend Pimalai, but don't know if you would be able to get it in your price range. Another good one is Lanta Seahouse which I have stayed at 3 times, and always have a nice time. Make you you make it into Saladan one night and go to Lanta Seafood for a great meal. You will have a great time in Koh Lanta if you are going there to chill out, if you are wanting to party I would go somewhere else as it will be very quite in September.