Ok Guys. I used to sell cars, so here's my take. If you ask to see an invoice, the dealer may show it to you. I did my homework before buying my Hummers so I have not seen one. However, on all the invoices I have seen, there is a charge for advertising.
So you ask, why is that? The answer is: you asked to see an invoice. A breakdown of the charges associated with the vehicle. None of this stuff comes cheap. Everything from the destination charge to emissions (if applicable).
Why should I have to pay for advertising? Isn't that GM's responsibility? Or even the dealer's responsibility? The answer is YES. Again, you asked to see a DEALER INVOICE. This is what the dealer is responsible for (ok not counting kick backs and the like). Think of it this way. You also pay for the lights at night at the dealership, those Big Ass Fans on the ceiling and the maintenance guy too. GM and dealerships pay for these things by selling you a vehicle. It comes out of your pocket. GM pays for advertising by holding dealerships responsible for their share. After all, it's not a Customer Invoice, it a Dealer invoice.
Anyways, knowing this does not really matter. There are plenty of costs to run a dealership. What matters is getting a great deal.
If you are just starting out, try starting here:
http://www.kbb.com
Remember to include all of the add ons to the invoice price including applicable emissions for your state etc.