Federal Tax Credits for Energy-Efficient Windows and Doors?

We are a residential contractor and have been receiving inquiries for replacement windows which meet the Federal Tax Credit guidelines. We are aware of the Energy Performance ratings required, and understand that the Tax Credit is only on the cost of the windows and not the installation labor. We buy from our supplier at a cost, and mark-up that cost to our customer during the sale. My question is: Does my customer get a tax credit on what my firm paid for the window ( before mark-up) or do they receive the tax credit on the marked-up cost for the window?