Agree with RND's points. To me, a 90k 100 series would be nice. Then it's just a function of how much cash I want to keep as my own. That's pure personal preference. I don't think it's good for everyone nor do I think it's a good blanket price. He points out the numbers for good reason. The price could very easily go down to book if the dealer wants to sell it bad enough. What's the downside is asking for 5% over book? None if you'll walk.
when negotiating, decide whether it's a win/win or zero sum situation. Will you ever see the seller again? Do you want a positive relationship or do you care?
And that's my main point. not trying to rehash any anti-car dealer sentiment, but negotiating with them is a zero sum proposition. Too many predatory tactics that have become the norm in the industry. Much of that gets back the way the industry is structured. The automakers count a sale when the vehicle is delivered to the dealer, not when the customer actually takes possession. They get the short side of that deal and end up holding a lot of automakers bad inventory, so they have to in turn push it onto us.
If I deal with a car dealer, I'll generally fax/email them a quote with specifics and price and tell them take it or leave it. If they go into sales mode and try to start making accomodations, I either ignore or brutalize them.
Do your research. Get firm on your price. Keep the negotiation rounds low. The second the dealer tries to make accomodations, walk. And I mean hang up or walk off the property. Make time your ally, not theirs.