http://www.spotrac.com/nba/miami-heat/cap/
So they got with Napier close to 3 million in salary between 3 players. That means a cap hold of 500,000 for the remaining 10 spots needed on the roster. That is 5 million dollars.
So in reality they have 55million in cap space to sign fa and 60 million if they sign enough players to reach the 13 needed to remove cap holds.