Security experts say there’s another way to reduce crime: Hire much more security, including more off-duty, uniformed police. At $35 an hour, a typical rate in many cities, 12 hours of police coverage a day would cost each store about $150,000 a year. Multiply that by the company’s 3,500 Supercenters—the largest stores and the sites of most of the serious crime—and it would cost half a billion dollars a year.