Comparison of Sustainable Transportation Act or the COST Act This bill requires the federal government to analyze the costs and impacts of replacing vehicles fueled by gasoline with electric vehicles or flex-fuel ethanol vehicles (e.g., vehicles capable of using fuel that contains a percentage of ethanol). Specifically, the Government Accountability Office must analyze the costs of replacing light-duty vehicles (i.e., vehicles weighing 8,500 pounds or less, such as passenger cars, minivans, and sport utility vehicles) in the federal fleet that are fueled with gasoline with electric vehicles or flex-fuel ethanol vehicles. The federal fleet means vehicles owned or operated by the federal government. In addition, the Department of Energy (DOE) must analyze the overall greenhouse gas impacts of the following types of vehicles: (1) a conventional gasoline vehicle, (2) an E85 (a fuel containing 85% ethanol and 15% gasoline) capable flex-fuel vehicle, or (3) a battery electric vehicle. In conducting the analysis, DOE must utilize the most recent Greenhouse gases, Regulated Emissions, and Energy use in Transportation model (commonly referred to as the GREET model) developed by Argonne National Laboratory.