What are you looking to buy? There isn't a lot of margin on the vehicles themselves. The dealer makes money on the finance and insurance contracts and vehicle accessories etc. That stuff is all high dollar.
They make the most money on the used.
However, there are some really big factory incentives from some manufacturers that make new cars really attractive. Buy at the middle of the month or very end. If they are close to their volume incentive, they may be willing to sell for a very competitive price as registering the sale may mean 10's of thousands in factory money.
Also, if you know wholesale on your, which is essentially what it's going for at auction, I wouldn't take less than that.