IAS 2 Inventories, para. 9 states that :
Inventories shall be measured at the lower of cost and net realizable value.
Para. 7 of IAS 2 defines the Net Realizable Value as :
Net realizable value refers to the net amount that an entity expects to realize from the sale of inventory in the ordinary course of business. Fair value reflects the amount for which the same inventory could be exchanged between knowledgeable and willing buyers and sellers in the marketplace. The former is an entity-specific value; the later is not. Net realizable value for inventories may not equal fair value less costs to sell.
So, net realizable value is the estimated selling price in the ordinary course of business less the estimated costs of completion and the estimated costs necessary to make the sale.
The utility of an item of inventory is limited to the amount to be realized from its ultimate sale; where the item's recorded cost exceeds this amount, IFRS requires that a loss be recognized for the difference. The logic for this requirement is twofold; first, assets (in particular, current assets such as inventory) should not be reported at amounts that exceed net realizable value; and second, any decline in value in a period should be reported in that period's results of operations in order to achieve proper matching with current period's revenues. Were the inventory to be carried forward at an amount in excess of net realizable value, the loss would be recognized on the ultimate sale in a subsequent period. This would mean that a loss incurred in one period, when the value decline occurred, would have been deferred to a different period, which would clearly be inconsistent with several key accounting concepts, including conservatism.
Revised IAS 2 states that estimates of net realizable value should be applied on an item-by-item basis in most instances, although it makes an exception for those situations where there are groups of related products or similar items that can be properly valued in the aggregate.
Recoveries of previously recognized losses
IAS 2 stipulates that a new assessment of net realizable value should be made in each subsequent period; when the reason for a previous write-down no longer exists (i.e., when net realizable value has improved), it should be reversed. Since the write-down was taken into income, the reversal should also be reflected in profit or loss. As under prior rules, the amount to be restored to the carrying value will be limited to the amount of the previous impairment recognized.
(Sources : Wiley IFRS 2008 : Interpretation and Application of International Financial Reporting Standards - Barry J. Epstein, Eva K. Jermakowicz and IAS 2 Inventories)