string old = "80.57K";
 RemoveCharfromString(old);
        public static void RemoveCharfromString(string old)
        {
            if (!(string.IsNullOrEmpty(old)))
            {
                Console.WriteLine(old.Replace("K", ""));
                Console.WriteLine(old.Remove(old.LastIndexOf("K"),1));
                Console.WriteLine(old.Substring(0, old.LastIndexOf("K")));
            }
        }

        static void Main(string[] args)
        {
            String old = "80.57K";
            Console.WriteLine(RemoveCharfromString(old));
            Console.ReadLine();
        }
        public static String RemoveCharfromString(string old)
        {
            if (!(string.IsNullOrEmpty(old)))
            {
                String newStr = old.Replace("K", "");
                double result = 0.0;
                if (Double.TryParse(newStr, out result))
                {
                    return (result * 1000).ToString();
                }
            }
            return old;      
        }