As per AS-2, Items of inventory are valued at cost or net realisable value whichever is lower.
Cost would include cost of purchase, cost of conversion and others cost incurred to bring the asset to its present location i.e to the point of sale or point of consumption.
Therefore, the custom duty paid will be the cost incurred for bringing the machinery to the point of sale and assuming that net reasiable value in your case of machinery is higher, such machinery will be valued at cost.
As per As-2, the entity must disclose by way of proper Notes to Accounts,
1.) The accounting policies adopted in measuring the inventories i.e cost or NRV
2.) The cost formula i.e FIFO, Weighted Average Cost or Specific Identification Cost (in few cases)
3.) The total carrying amount of Inventory and it classification appropriate to the enterprises.