If they promised you a price and didn't honor it, the "grand scheme of things" is their dishonorable business practices. In the grand scheme of things, walk out. Every time we the consumer choose to "just pay the extra because, well, it's only X dollars" you teach them to do that to the next guy b/c they will likely get away with it. Don't let emotion run the negotiation! If your car caught fire in their lot and you needed a new car that day, I get it. That $500 is chalked up to bad circumstance and they got you. However, under any normal circumstances you should stick to your guns. $500 buys a whole lotta lotta chedda.
1) It's a set of new tires.
2) It is a set of roof rails, racks and all weather mats
3) It is a remote start option
4) etc