With AVCO or FIFO costing, cost are modified automatic with goods receipt.  Coming from the price on the purchase order.  In V16 the cost price is also modfied from the Vendor bill. (I am not sure how that work in V15). You can check what has happened in the report Valuation in the Inventory app.  From this report it is possible to enter a manual correction.

If this is a manufactured product, the cost is coming from the manufacturing order.  If you use AVCO on the finish product it is recommended to have same costing method on components.  

Check your product costing method, if it is not 'standard costing', product cost will change according to inventory qty(if it is average costing), in FIFO LIFO costing method also cost will automatically change.