Use
CarsDirect.com -- America's #1 way to buy cars online! to see the invoice. Once you've plugged in your zip code, the make/model and options, it will give you MSRP and invoice. This is the official dealer's "cost" for the vehicle.
However, even if they sell the vehicle at this price (or even less) they still make money on the sale through holdback and floorplan. It's not their TRUE cost to purchase the vehicle. You should be able to get a little below invoice, then apply the rebate or financing.