Several prominent websites offer car dealer invoice prices. Edmunds.com is one source; CarsDirect.com and TrueCar.com offer a similar service. In each, you type in the make and model of the car you're interested in, along with any options that would add to the price. With that information, these sites present you an invoice figure. Failing that, you can always ask the dealer to show you the invoice. Many will freely do so, because that doesn't reflect what they actually paid.
Some costs aren't included in the dealer invoice. These include any tax, tag, title, license advertising or registration fees. The destination charge sometimes is included in the invoice, but not always. Similarly, advertising fees may be on the invoice, which indicate that it's a charge by the car manufacturer. It also can be tacked on by the dealer to offset its own costs. If it's the latter, you may be able to negotiate that fee down or eliminate it.
Discounts Not Included
The dealer's cost often is lower than what the invoice would suggest. Dealer incentives can be offered by the auto manufacturer to sell a particular model, reducing the cost of the car. Many manufacturers also offer their dealers a holdback, refunding some of the money the dealer paid for the car at the point of sale. Holdbacks are particularly hard to determine, though it can be a useful concept to broach if a dealer complains that she has to sell the car over invoice to make a profit.
When negotiating the price of a new car, visiting car-buying websites like Edmunds and TrueCar can alert you to the known dealer incentives. Generally, older vehicles that both manufacturers and dealers want to move off the lot in favor of newer models have greater incentives and holdbacks associated with them. While getting the exact amount of a dealer cost for a particular vehicle at a specific location may be too challenging, it takes far less work to at least know what the invoice and incentives are before making your best deal.