Item Costs
You can set initial costs for items by going to Admin -> Inventory -> Item Beginning Balances.
You can make cost adjustments to items by going to Inventory -> Items -> Item Cost Adjustments. You can adjust a single item, or you can apply the total from an AP bill against items received on a purchase order (Apply Total From AP Bill and Against Items Received On PO fields). Applying the total of an AP bill against a group of items can come in handy when you need to spread freight costs to items to have a landed cost for those items.
With the Corporate version you can also adjust item costs using the Inventory -> Items -> Export/Import Edit Tool when the add-on Update Inventory Costs & Quantities With Export/Import Tool is active. With this method you can export a CSV file with current inventory costs and reupload the updated costs after making changes. GL entries will be made automatically to account for the change in cost values.