Figure just the island will cost you between 10-30 million depending on the location and features of the island. Is there reliable water available? Is there a sheltered bay? What quality sand are the beaches made of? Are you close to a drug lord's island?
Take the cost of whatever building you're planning on putting together. Add between half and double that cost to the total cost of building. You're paying for transport of materials and challenges that will inevitably rise due to the proximity of the sea.
You will also want to hire a caretaker. Maintenance on an island residence is a bitch.