One night on the island will cost you any where from 1000 U.S. dollars to 25,000 US dollars. Parrot Cay is often regarded as the nicest hotel in the Caribbean, and one of the nicest in the world.
Don't expect world class water park and you'll have fun. There are slides, a roller coaster, and a lot of water. Entrance fee isn't high, bathrooms have showers, you can relax on a long chair on a grass and sunbathe if you're tired of water.