using static System.Console;

using System.Globalization;

​

public class Program {

    public static void Main() {

        var valor = "9.5789";

        WriteLine($"Custo: R${valor:C2}");

        WriteLine($"Custo: R${valor:C2}".ToString(new CultureInfo("pt-BR")));

        WriteLine(string.Format("Custo: R${0:C2}", valor, new CultureInfo("pt-BR")));

    }

}