My favorite was getting the goat eyed stare from somebody I had just gone over the invoice on a new car with and showed them what we paid to the manfacturer to get the car on the lot. They would still insist that we could sell a car we paid $28k for to them for $18K.
At times you can do this, if you're purchasing intelligently and using leverage.
Four years ago when I purchased my truck, I wanted it brand new and wanted the best price - yeh, we all do. So I bid dealers against each other - a fairly common practice now - if you know exactly what you want to purchase. You are in control and should be. Because of all of the promotions and discounts and bonuses that manufacturers make to dealerships, there are times where the dealership makes far more $$ on the manufacturer promotions than they do on the car sales.
For instance, Honda might tell a Honda dealer that if you sell 100 Hondas this month (any flavor), the dealership will get a $100,000 payment from Honda. It's June 27, the car dealership's sold 90 cars, and they are going to sell 10 more cars to get that $100K or else, including selling cars at a loss. Who cares? There's big money waiting. There's all sorts of these programs, and you just have to sniff them out. And you do that by bidding dealers vs. each other. A dealer that's not close to that 100 car bogey won't sell at a loss. Those that are will.
I paid for information about the make/model of my vehicle, so I would know what the invoice and retail and other detailed #s were on my model, including what special programs the manufacturer was running for their dealerships at the time. I then used the power of the intrawebs to figure out, within 100 miles of me, which dealers had my truck sitting on their lot, in the color/style I wanted. Then, on a planned day (a Monday) towards the end of the month (by design), I contacted each dealership in writing (email) and told them
"Hi, I'm Tim, and tomorrow at 3PM, I'm selecting the winning dealership to sell me this truck. I will then purchase the truck, after receiving the buyers order, within 24-48 hours. I'm going to be buying this model truck with these features. I am letting you know that I am contacting you and 7 other dealerships. Since the end product is the same, I am making this purchase decision based solely on price. Please submit your price to me, if you are interested in winning my business, by (stated time) tomorrow when I'll make my decision."
I got some 'no responses,' some quotes that were $3K to $8K over invoice, etc. I was losing faith that I'd hit the market at the right time, that maybe there were no promos going on, etc, until like an hour before I was closing my bidding I had a dealer call me. He was willing to sell me the truck for $1,000 under invoice (I know this doesn't necessarily translate into what the dealer actually paid) and per my word I purchased the vehicle within 2 days.
I've known some people to go $3,000 or more under invoice. Sadly for me, there were rumors of my make/model going off the market soon. I live in Washington DC and from Philly to western MD to Richmond VA, there were only 7 of these trucks with my desired features available in the whole region. If you play this game with a make/model when there is a lot of inventory, you can get well below invoice. Some dealer, somewhere, is looking to sell at a loss, nearly all the time.
