In Product category- check the following

Costing Method - select your method

Inventory Valuation method  -  Automated - this option calculate automatically change the cost

Remember if you change it will affect all the product under that category.

Or.  Standard Price means you estimate the cost price based on direct materials, direct labor and manufacturing overhead at the end of a specific period (usually once a year). You enter this cost price in the product form.
After you can inventory valuation to unaffected by other items create another category and assign to this product..only. later the valuation with new cost you can change the category back.

Hope you got it