On my invoices for windows I will take the total cost and divide by half or what I paid, which ever is higher. The customer is paying ME for the windows, they are not paying my supplier. Therefore I deserve a markup and so forth for the transaction. Yes I happen to be a middle man when it comes to supplying materials but they are still paying what ever ammount I think is fair.
This actually will work in the customer's ffavor. It's like on shingle manufacturer waranty's when they ask for labor and material costs. I do the same thing. Take the total price and divide by half. I don't even give it any real though, it's not worth the time. And again, legally they are buying the materials from me not my supplier, and it's logical I would charge a markup.