Use the vehicle search on these two sites:
http://www.goodsamautobuying.com/new_cars/default.aspxhttp://www.truecar.com/They show the "invoice" price as best as we will ever know.
As noted in the fine print at the bottom of the page:
"Factory invoice may not reflect dealer’s actual cost for a vehicle. The invoice may be higher than the true, final cost to the dealer. Factors that may affect the dealer’s actual cost, include but are not limited to, holdback, factory-to-dealer incentives, advertising fees, reserves and other charges, which may vary by vehicle."