IAS 2 inventories: lower of cost and NRV

Equipment constructed for a customer for an agreed price of $18,000. This has recently been completed at a cost of $16,800. It has now been discovered that, in order to meet certain regulations, conversion with an extra cost of $4,200 will be required. The customer has accepted partial responsibility and agreed to meet half the extra cost.

sir with regards to the above point my text states that NRV will be $15900(which i concur with) however then they go on and state that inventory will have to be written down by $900.

I wanted to ask why do we not increase the costs to 16800+4200/2=$18000 and then compare with $15900, to get a write down value of $2100?