Relevant Costing

Hello Sir, Following is the question I have a doubt in: A company needs 400kg of material Z to fulfil a customer order in one month’s time. It currently has no material Z in inventory. The current purchase price of material Z is $20 per kg and this is expected to rise to $24 per kg in one month’s time. Material Z is perishable and normally 20% of stored material is lost per month. The company expects to have 200kg of material Y in inventory in one month’s time with no alternative use other than to sell it for scrap for $18 per kg. The 200kg of material Y could be converted into 200kg of material Z in one month’s time at a cost of $4 per kg. Material Y is not perishable. In the answer, The total relevant cost is (200kg x $22) + (200kg x $24) = $9,200 In the $22, they are even added $18. I did not understand why are we accounting for the scrap value when we can covert Y into Z at $4. Thank you in advance!