Bonjour Marquis,
Where do you plan to stay in Phuket? Patong or quieter area (Kata-Karon..) or very quiet area (Rawai, nai Harn...) ?
Do you plan to bring some girl to your hotel ? If yes, many luxury resorts will charge a joiner fee (500-1000 bath a night), so it would increase your budget.
I wonder if it wouldn't be more intresting to rent a nice appartment for 1 month even if you don't stay so long, all depends on the average budget you want to spend, I think.
IMHO, I wouldn't book on Internet, but I would directly call the resort-hotel-renter. You will probably get a better price if you deal a bit.
If you can give us more precisions, it will be easier to help you in your seek
