using System;

public class Program

{

    public static void Main()

    {

        Console.WriteLine("A: " + AddCommaToAmount("1.00099"));

        Console.WriteLine("B: " + AddCommaToAmount("1.099"));

        Console.WriteLine("C: " + AddCommaToAmount("1.99"));

    }

    public static string AddCommaToAmount(string amount)

        {

            double formatAmount;

            double.TryParse(amount, out formatAmount);

​

            return string.Format("{0:#,##0.##}", formatAmount);

        }

}