My suggestion is well within your budget - The Karon Beach Resort which is the only hotel actually on Karon Beach. It is only a 3star but is very comfortable. The other hotel I like is Le Meridian Beach Resort which is on is on its own private beach just south of Patong. Sorry have no personal experience of staying at hotels on Kata but have heard that the Kata Thani is nice and there is always The Boathouse but I think it is too built up around it now.