Dealers are only interested in the amount of profit they will make on a sale. Every vehicle has a Manufacturer's Suggested Retail Price (MSRP, aka sticker price, moroney, etc.) and an invoice price (the theoretical price the dealer pays for the vehicle). The difference between the two is the "markup or profit margin." If they sell it less than the MSRP, they have discounted the price. Dealers generally negotiate a deal based on the amount "over invoice," i.e., if they want to make $500 profit, they sell it for $500 over invoice, etc. The Edmund's homepage Tom referred to should tell you what the MSRP and invoice are of most any vehicle, based on the last time I viewed it (though it isn't always up to date). You can also buy "Edmund's" new and used car price guides at many book/magazine stores, that have the up to date prices (Edmund's would rather sell you the information as opposed to give you the information free on the Internet).
To complicate the matter, you will also hear about an amount referred to as "holdback." Holdback is an amount referenced on the actual invoice. This is generally about 3%. Manufacturers give the dealers a check quarterly for this amount. This amount is what the dealers use to cover their overhead (and maybe a little profit, depending on the overhead of the dealer, though they generally won't even discuss the holdback when negotiating a price for a car).
The last few cars I've bought for myself and friends I've bought over the phone. If you do the research and know what you want to buy, which options you want, and have determined what the invoice should be, you are ready to start. Many dealers in larger metropolitan areas have advertisements (in the used car section oddly enough) for fleet sales. When you call those numbers, tell them what you want to buy and ask them if they have a vehicle like that and how much over invoice they will sell it for. If they tell me they want $500 over invoice (about the going rate for a vehicle anybody would want), I generally say something like "I can buy it in the Bay Area for $300 over, can you meet that?" After you've done this with a few dealers (generally the ones you've heard of that give the best deals), you can be fairly sure that you're getting about the best price possible. If you buy a current model year vehicle that's selling at a normal pace for less than $200 over invoice, you got a good deal! And make them show you the real invoice! And don't negotiate based on your monthly payment (email me direct, I'll tell you why)!
Of course if the manufacturer is giving the dealer an incentive to move cars, the dealer can sell the vehicle for less than invoice. This often happens at the last of a model year, or when a particular model isn't moving well. Edmund's seems to have good information on which manufacturer is giving what kind of incentives or rebates. Of course if a manufacturer is giving a $1000 rebate to the dealer, you'd want to negotiate from a price that's $1000 less than the usual invoice price. Or if it's the end of a model year and the new model year is already being sold, or if it's a vehicle that's been put into "demo" (short for demonstrator) service, you can deduct another 5% for either of those (approximately). The manufacturer gives the dealer rebates for those. Oddly enough, if you start discussing these issues with the dealer, especially over the phone, it's amazing what I've had them volunteer, e.g., "The manufacturer's giving us an X% rebate right now to reduce inventory, or get rid of last year's model."
And then there's the profit they make on financing, extended warranties, alarm systems, stereos, rustproofing, etc. Because the above information (actual dealer cost) is so easy to get, and today's buyers are so knowledgeable of dealer cost, dealers generally make more profit on these other items than the new car itself. You're generally better off to arrange your financing elsewhere (unless the dealer can offer a lower rate than your credit union), and negotiate the Hell out of the amount they want for extended warranties, or any addons. For example, it's not uncommon for a dealer to want $800 for an extended warranty, and if you negotiate you may get it for $300.
And then there's what you're going to get for your trade in. Four words...don't trade it in...unless it's a piece of crap and you don't want some private buyer coming back to tell you. The dealer will only give you wholesale price for a trade in, because they have to resell it and make a profit. The used car lot has to make a profit on what they sell too. But remember, that wholesale amount is negotiable too!
Another thing to remember, if a dealer is selling you a car without air conditioning, and you want a/c, they make the profit on the car they sell, the parts department makes it's profit on the air conditioning kit they sell to the service department, and the service department makes it's profit on the installation. That's 3 markups. If you buy the car that has factory air, you only pay the profit on the car (and you don't give the service department the opportunity to screw up the installation)! Sames goes for all other add on accessories.
Based on the current Edmund's figures, a 1996 Coupe costs the dealer $32,432 (without any options). My guess is that a dealer's cost on a leftover 1995 Coupe (without any options) is around $28,000. Trust me, I've seen few examples where the dealer didn't make a profit. So, your job, if you so choose, is to determine how much the car is costing them and how much profit they're making.
Hope this was of some value. And believe it or not, this was the condensed version...I didn't even talk about buying used cars. Actually, I've been sitting on the other side of the table (purchasing/contracting) for the last 20 years. I spend your tax dollars. I hope this doesn't get me as much wrath as being an attorney did a couple of other people.