How is the margin calculated if the purchase order is created after the initial allocation of item cost price? Will it use the cost price of the initial item or the cost price of the purchase order?

For example, suppose I add an item with a cost price of $100 and 100 stock units. After one day, I create a purchase order and bill with a cost price of $150 for additional units of the same item. Will it retain the cost price of the initial item or use the cost price of the purchase order and discard the initial cost price?