Nath is spot on (surprise surprise
) The bargaining power is in the add ons.
You can argue the cost of a car all you like with a dealer they know what they are prepared to drop to for the basic car before you walk in the door and that price wont change dramatically between dealers. A salesman is not there to move add ons he's only there to move cars he doesn't give a shit about the cost of addons it's a separate department and he needs to show each car going on the door at a cost not including the add ons.
They will also tell you how you have bargained their left butt cheek off and their third kid will be going without school shoes this year because when the boss sees the deal the salesman made he will have a tanned arse.
So once you get a price for the car you then play with their fanciful figures for addons. Don't be ridiculous and tell them you want $1500 worth of stuff for nothing because even through the buying public is suppose to be too stupid to realise that they could easily afford to do such a deal it's a game you can't win.
Once you have their costs of the add ons add half the cost they claim them to be worth to the lowest price of the car and then tell them that's your offer. They will then counter, you stand firm, they will then try a sob story on you and go and see the dealer principal. The salesman and the principal talk shit in the hope you think they are talking about your deal, the salesman comes back with a counter offer. If it's close to your offer tell them you aren't changing your mind. If you wish to play the "dealer principal" game again feel free but it's not necessary. If you are arguing less than $1000 get up and walk out if you make it to the door and they haven't called you back go to the next dealer, but chances are for less than $1000 they wont let you get to the door. Salesman hate seeing the backs of people so once you head for the door you are a loss so stand firm you'll get your deal.
If the first price you come up with gets met with reluctance and the salesman doesn't even try the "dealer principal" game walk out and reassess your price at the next dealership because it was unrealistic.
Bargaining with sales droids is a game you can never win, you can only draw but it really pays to play the game and atleast ensure that it's not entirely a win for the salesman.