Hmmm, sorry but I believe there's a reasonable markup on new cars.
Dealers are prepared to drop their prices significantly all the time. You don't get large knock downs on low sale goods with tights margins.
Dealers make their money in three ways:
1. Margin on the car (normally 2% for a basic model)
2. kick backs from the manufacturer for hitting targets
3. Aftersales, service, parts, etc
The kick backs can be quite large, so if a dealer only needs to sell two models to benefit from a 5% bonus, he might well discount the next two cars and sell at a loss to hit the target and make the months / quarter's numbers.
Quite often the manufacturer will offer discounts to dealers to shift stock, so if you haggle and get 10% off, most likely there is a promotion on and the dealer is also getting 10% off the price pay for their model.
NB My wife manages franchised dealers for car manufacturers for a job. She says no one would chose being a dealer; you'd make a better return putting the money is a savings account. It's a very tough business now as over supply and competition have killed all the profit in the retail side.