I just talked to a former car sales manager who confirmed what I'd read somewhere... that the run of the mill ordinary cars can sell for as little as 200 over the invoice.
So check on what the dealership has to pay for the vehicle, add a couple hundred dollars, and make that your final offer. You know you'll have it back to them for servicing for all the recalls that will happen, and the scheduled maintenance to get your extended warranty... so why not buy it for the least possible?
On the expensive or premium vehicles that they aren't offering discounts, rebates, etc etc, you'll have to pay about a thousand over invoice. A Cadillac for example may have the $1,500 over invoice rule of thumb in effect, and a Corvette a $4,000 over invoice. Ask the salesmen at several dealerships and get the most opinions possible before purchasing.
The dealerships make more from the service department that the car sales. That's where the money is, fixing the new car they just sold you. I think it's a crock that they sell you a new car, and expect it's enough of a hunk of junk that they'll soak you for more money to fix it until it works as they've promised it would when they sold it to you... just read Automobile magazines reviews of the fleet of different cars they use, and how many repairs they have to put up with on the new cars and suv's they have. Ridiculous. See the Upshift section of Automobile magazine, in the page 90-100 area of the magazine, or read reviews like this that don't have the same sidebar info as the magazine but give you the same realization the new cars have lots of issues that you'll be bringing it back to the dealership to fix http://www.automobilemag.com/reviews/sedans/0512_volkswagen_phaeton/problems.html
