Hi Marcelo,
From the Add / Edit Work Order screen, add the job item to the Job Item Line grid and click on Details... for the item. Here you can set the Cost of the Item for the purposes of Job Costing, just be sure to set the Amount to 0 and it won't be charged as part of the price for the work order. You can also specify on this screen whether or not to include the item when invoicing.
This can also be done directly from the Line Item Grid, which is useful for streamlining the process if you have multiple line items to do this with. To do this, add the Cost Per field to the Job Item Line grid by clicking the gear icon in the bottom-right corner of the grid, checking the box next to Cost Per and clicking the Update link below. This will allow you to edit the cost of items directly in the grid, just like in the screenshot below.
Hope this helps.
- Justin