The cost pulls from the Products Tab UNLESS Inventory Cost is turned on in Settings > Inventory Settings.
If inventory cost is turned on, the COGS (Cost of Goods Sold) in the Product Mix Report pulls from the cost in the inventory and the cost calculated by the cost of ingredients. It does NOT pull from the cost set in the products. Similarly, the cost in inventory will not pull from the cost set on the product page. It either needs to be set in inventory or calculated through ingredient level tracking.