A beautiful C# logical

A TextBox in the web page(It will show decimal), data store in database(no decimal). if input nothing, I will give the value "999999" to the database, if the input value is "4.56", I will give the value "456" to the database.
when I get the value from the database, if the database value is "999999", the TextBox will display null; if the database value is "456", then the TextBox will display 4.56".
That is the function:

Code:

        public static string TotalBaselineMaximum(string MaximumAMT)
        {
            string tmpMaximumAMT = "";

            if (MaximumAMT == "")
                tmpMaximumAMT = "999999";
            else if (MaximumAMT == "999999" || MaximumAMT == "0")
                tmpMaximumAMT = "";
            else
            {
                decimal dec;
                dec = Convert.ToDecimal(MaximumAMT);
                if (MaximumAMT.Contains("."))
                    tmpMaximumAMT = Math.Round(dec * 100).ToString();
                else
                    tmpMaximumAMT = (dec / 100).ToString();
            }

            return tmpMaximumAMT;
        }