10000000 -> 1000,000,0

  public string FormatAmount(decimal? amount = null)
        {
            amount = amount ?? Amount;

            var s = amount.ToString();
            if (s.Length < 2)
            {
                return s;
            }

            var len = s.IndexOf('.');
            if (len < 0)
            {
                len = s.Length;
            }
            if (len < 4)
            {
                return s;
            }

            for (var i = len-3; i > 0; i -= 4)
            {
                s = s.Insert(i, ",");
                i++;
            }
            return s;
        }