Commodity Tax Act Article 16

Where the selling price of any taxable commodity is not available for the current month thus the taxable value can not be calculated in accordance with Article 13, the taxable value of such commodity shall be the last or most recent month taxable value. If there was no taxable value last month or in the most recent month, the taxable value shall be the taxable value of most similar commodities. If there is no similar commodities for newly produced products, the taxable value shall be calculated first based on the production cost plus profit and shall be adjusted according to the selling price after the commodity is sold.
中文原文 貨物稅條例第16條
產製廠商出廠之貨物,當月份無銷售價格,致無第十三條規定計算之完稅價格者,以該應稅貨物上月或最近月份之完稅價格為準;如無上月或最近月份之完稅價格者,以類似貨物之完稅價格計算之;其為新製貨物,無類似貨物者,得暫以該貨物之製造成本加計利潤作為完稅價格,俟行銷後再按其銷售價格計算完稅價格,調整徵收。

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