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.
In the United Kingdom it is easy: 1 mi = 5280 ft (by definition, so exact). In the United States, there are two kinds of miles and two kinds of feet that are relevant to this question. For both miles and feet, there is a pair of international units and a pair of survey units.