Quick answer: Phuket runs about EUR 70 per day mid-range - noticeably less if you eat local and walk. Cheapest window: November-April - the dry season with calm seas; May-October brings rain, swells and red flags on west-coast beaches.
What things actually cost
street-stall pad thai
2 EUR
beer from 7-Eleven
1.50 EUR
beer in a beach bar
3 EUR
scooter rental per day
7 EUR
one-hour Thai massage
9 EUR
Phang Nga Bay group tour
40 EUR
Eat well for little
Pad thai or basil chicken from a street stall - about 2 EUR
Tom yum goong at a local restaurant - about 4 EUR
Moo hong (Phuket-style braised pork belly) in Old Town, e.g. at Raya or One Chun - about 6-8 EUR
Fresh grilled seafood at the Rawai seafood market stalls - pick your fish, about 10-15 EUR
Mango sticky rice from a market stall - about 2 EUR
Roti pancake with banana from a night-market cart - about 1.50 EUR
Money mistakes to avoid
Renting a scooter without a motorcycle license and helmet - insurance will not cover you and police checkpoints fine tourists daily
Taking unmetered taxis or tuk-tuks without agreeing a price first - use Grab or the Smart Bus instead
Booking a hotel in Patong by default - most of the island's charm is in Old Town and the south
Swimming past red flags in green season (May-October) - rip currents on the west coast are genuinely dangerous
Transport without the tourist tax
From the airport, the Smart Bus runs down the west coast for about 2.50 EUR, a metered taxi to Patong or Kata costs 18-25 EUR (45-60 min). On the island use Grab or rent a scooter for about 7 EUR per day; there is no real public transport between beaches.