From the coastal bluffs of Newport Coast, CA , to the private shores of Fisher Island, FL , America’s priciest ZIP codes have much in common: sprawling estates defined by privacy, panoramic vistas, and a rarified buyer pool with budgets vast enough to afford the eight-figure price tags. Economists at  Realtor.com®  recently identified the 10 most expensive communities across the U.S. To do that, they focused on areas that had at least 30 active listings in January. Among those qualifying ZIP