Be default, the price, or cost of the product, on the receipt is pulled from the Purchase Order. You can receive the product at a different cost here, if you wish. This will not update the Purchase Order. You will need to update the Purchase Invoice approved price to match that of the receipt. Otherwise, when you post the Purchase Invoice, you will receive a message stating the receipt will be changed to reflect the Purchase Invoice price.
If no Purchase Order has been created, then the Unit Cost will be pulled from the Last field, which is the Last Cost the Product was received at, on the Inventory -> Management tab of the Product window.