For those who purposely dress down, I would encourage you not to do that. They will think you don't know anything, and will work you worse than if you come in knowing what you are talking about, and dressed nicely.
That being said, they should treat you no different regardless of how one is dressed.
For those who have no choice but to dress down, who are broke, whose credit is shot, they always pay the most. Don't think for a second that the dealer doesn't want to see them in the showroom.
The educated buyer, who can dress up if they so chose, who make some money, who have good credit, can usually go and get a good deal. They are not the ones who get taken just because they can afford it.