Relevant Material cost

I came across this number in the revision kit on open tuition where by a contract requires 200kg of material X.The company has 500kg in inventory which originally cost $6 per kg. Material X has no other use and if not used on the contract will be scrapped for $2 per kg. What is the relevant cost for the material.

Though the answer has been given as $400 I still dispute it because I thought if 500kg are already in the store we are gonna use 200kg the balance will remain 300kg which we shall scrap @2kg hence 300kg x 300=$600

reply will be highly appreciated. Thanks