Cheap is a relative word when talking about the Bahamas. That being said, it's my experience that the least expensive airfares, are available by flying into Nassau. More flights, and more carriers servicing Nassau.
The mile is an English unit of length of linear measure equal to 5,280 feet, or 1,760 yards, and standardised as exactly 1,609.344 metres by international agreement in 1959.
There are 27,878,400 square feet in a square mile. That is the correct amount because to find the area of something, one multiplys the length and width. So to find how many square feet are in a square mile, one must multiply 5,280(the amount of feet in a mile) times 5,280(the product is 27,878,400). I hope that answers your question :)