Phuket gets a bit expensive compared to other cities in Thailand. I found that even Bangkok wasn’t as expensive as is Phuket. So, at least make up your mind that you will have to be spending more in that island.
Transport is also quite expensive in Phuket. A motorbike-rikshaw (or motorbike taxi) will cost about 60 baht on an average. Rikshaws are even more expensive, a 3-km ride on a tuk-tuk will cost you about 320 baht. Rides on songthaews are between 25-50 baht. The island isn’t that huge, my advice is that you rent a bicycle and roam around, or just walk the distance if its only within a kilometer or two.
If you were to stay in a hostel, eat your meals from the street stalls, cooked a lot of your meals and had a fancy dinner only once or twice a week, used local transport options, then your daily budget in Phuket will be between 1,050 baht and 1,450 baht. ($30 USD and $40 USD)