Even selling the vehicle at invoice the dealer makes money. Especially if they floorplan the vehicles. There is a lot of money on the backside most car buyers never know about.

When I bought my Focus it was just shy of 9k and the sticker price was 15k. Sold it to me at invoice, I got the hold back, ad money and what ever else they gave up. I worked there and knew it had sat on the lot for 9 months.

That's another thing, try to see how long the car has been there. Older units sell for less.
The end of the month is a true fact. If the dealership is short that month there's a good chance you can get a good deal.