Thanks for updating the thread, @deltabird,

You can directly assign the cost to the item used for billing. However, costs (Expense) will only reflect when you create expense transactions. If you mark the item as billable, the system will assign it to the customer transaction with the Sales Price (Income).

This is the normal process in QuickBooks for billable transactions. The level of detail in your Item List will determine the preciseness of your job cost reports. Learn how to add, edit and delete items in QuickBooks Desktop.

If you need to use the cost on both on the expense and on invoices, you can simply update the amount as you create the transactions.

If you need further clarification about this process, let me know in the comment below. I'll be right here to share some more insights to help you. Have a good one!