The customs value of imported goods subject to ad valorem duties shall normally be determined and calculated on the basis of the _______ value.

transaction value 是「交易價格」，即進口貨物出口至進口國銷售時，買方實際支付或應支付的價格，並依規定作必要調整。這正是WTO《關稅估價協定》第1條所採的首要估價方法，最符合題幹中 customs value of imported goods subject to ad valorem duties shall normally be determined and calculated on the basis of the _______ value 的語意。