You will find easily a nice appartment for 8.000 bath/month, but a small one. If you want to rent a house, check on
www.houseinphuket.com
All depends also on where you want to stay : Patong is rather expensive, as well as Rawai (many expats there). But if you don't want especially to live close to the beach, you will find many houses around Chalong, intresting area as it is a central point to Patong, Phuket Town and Rawai-Nai Harn IMHO.
Where will you work in Phuket ?