1) Click on 'Manage Inventory' under the 'Inventory' heading in the Control Panel menu.
3) Click the 'Edit' icon at the end of your item to go to it's settings page.
4) In the right column, edit your item's cost in the 'Cost' textbox.
5) When you are finished editing your item's cost, be sure to click the 'Save' button at the top to finalize.