It all comes down to how expensive the car is, what type of car it is and who and where the dealer is and even what time of year it is.
Cheap, mass market models have smaller margins while expensive and niche model cars tend to have larger margins.
If its a new car you may be able to save a bit if you shop around. Often dealers will make little or no profit on a car to shift old stock and then they recoup the costs through servicing.

New car prices are very negotiable. You just have to be good at it and willing to sign on the line. A sales guy isn't going to "drop his pants" for everyone who comes along. Basically if you can get the price down to what you want, then there should be nothing stopping you from completing the transaction.
I bought a VX SS (Series II) in December. I paid $45k drive out the door (RRP is $48,100 plus dealer delivery, reg and stamp duty). And I never visited the dealer I dealt with them via email!!! Gave them a credit card deposit. And went in a week later to pick up the car.

I have found secondhand sections of car companies (eg. Ford, Holden), don't seem to flex much with prices, where as 'secondhand only' companies will drop a fair bit off the price after a bit of haggling..
For example.. I was looking at purchasing an EA Falcon quite a few years back now, it was from a secondhand only dealer, they had $15,000 on it, and they came down to $12,000.. although I didn't end up buying it.. From there I went to the secondhand yard of a Ford dealer and purchased a Skyline, they had $11,900 on it, and the lowest I could get them was $11,000, and this was a cash buy with no trade..

Haven't actually brought a new car, but as far as I am aware, they won't drop the prices too much either, although haggle for as many 'extras' to be thrown in as you can, like headlight protectors, floor mats, seat covers, rear spoiler (if an option), roof racks (if you want them), tow bar to name a few.

Certain brands are easier to haggle than others, for example, Subaru was pretty stiff from my own experience while Ford and Honda seems more relaxed with Nissan right in between. Ask Hatzolah for advise how to twist the dealer's arms as he's such an expert (go Hatz..) Other than that, it's really up to your skills in haggling.

For second hand, *always* ask "how much will it cost me?" rather than asking for the cars price and the trade in. IOW, the bottom line.
After all, that's all that really matters, right? Not only that, but you can ask them to put a lower value on the car & your trade and pay less GST & stamp duty.

The same is true for new, but only if you have a trade-in. With no trade on a new car, there is less room to move, but it still pays to shop around - some dealers want to move "old" stock, especially tarted up showroom floor & demo stuff, or even to get their volume up for a better deal from the manufacturer. When I bought the Telstar, I saw the paperwork and the dealer made $100 on the car - yeah, I knew the sales manager, but he told me the owner wanted to sell more cars and that was the main reason I got such a bargain.

For second hand cars it probably depends on how optimistic the sticker price is. I was at a Holden dealership on the weekend looking at a VTII with 39,000km that had a asking price of $28,800 (yikes!) I got talking to the sales guy and within 5 minutes was down to $22,000 including Berlina alloys. Personally I thought that was still a tad too much. In contrast a similar vehicle was advertised elsewhere for $22,990 and I only talked them down to $22,000.
I try to find the wholesale price for the vehicle and add a $5,000 margin to get an idea of the dealers price.