As part of the 'Goods In' process, updating the supplier cost price is an essential part of the process. It will help you to maintain accurate average weighted cost pricing, which in turn means that your inventory valuation will be as accurate as possible.
The supplier cost price can be adjusted when generating a Purchase Order, or when receiving product at the Goods In section. This depends on whether you have the true item invoice cost available before placing the order. Some orders are received with a dispatch note rather than the invoice. If this is the case, the order confirmation should be obtained or a copy retrieved from your suppliers B2B so you can accurately update the Purchase Order values.
The key thing to remember is to make sure that before you commit your goods into stock, the cost price should be checked and adjusted where needed.
If updating the cost price at the point of creating the purchase order, click the Cost field in the Purchase Order and update the value:
Or at 'Goods In' when receiving products:
Changes to cell values are highlighted in Green.
The Average Weighted Cost is explained HERE