Customs Act Article 35

If the customs value of the imported goods cannot be determined under the provisions of Articles 29, 31, 32, 33, and 34, the customs value shall be determined by Customs using reasonable means on the basis of data resulting from an investigation by Customs.
中文原文 關稅法第35條
進口貨物之完稅價格,未能依第二十九條、第三十一條、第三十二條、第三十三條及前條規定核定者,海關得依據查得之資料,以合理方法核定之。

English translation for reference only; the Chinese text prevails. Not legal advice.