private static StringBuilder ret = new StringBuilder(13);
public static string FormateGoldString(string gold)
{
    ret.Remove(0, ret.Length);
    int length = gold.Length;
    string result = string.Empty;
    if (length > 2)
    {
        ret.Append(gold.Substring(0, length - 2));
        string str2 = gold.Substring(length - 2, 2);
        if (str2 != "00")
        {
            ret.Append(".");
            if (str2[1] == '0')
            {
                ret.Append(str2[0]);
            }
            else
            {
                ret.Append(str2);
            }
        }
    }
    else
    {
        if (length == 1)
        {
            if (gold == "0")
            {
                ret.Append(0);
            }
            else
            {
                ret.Append("0.0");
                ret.Append(gold);
            }
        }
        else
        {
            ret.Append("0.");
            if (gold[1] == '0')
            {
                ret.Append(gold[0]);
            }
            else
            {
                ret.Append(gold);
            }
        }
    }
    return ret.ToString();

}

很简单的就是分析各种可能出现的情况，将小数部分提取出来，加个点连接起来。