Here is an article from Clark Howard's site, that pretty much explains why dealer invoice pricing is not really a way to tell just what the dealer is making. When I bought my truck I got it for around $3500 below the MSRP, but have no idea if that was a good deal or not. I do know that it was the best deal that I could get out of the 4 dealerships I visited.
I bought my truck at the end of the month (on the 27th), and I know that part of the discount I got had to do with moving the truck before the end of the month to help the dealer meet a quota, that would give them a big bonus. The manager showed me the invoice, and showed me supposedly exactly what they paid for the truck, and supposedly exactly what they were making on the truck including $600 for the sales person, but of course this didn't include all the behind the back money they get that nobody knows about.
It is exactly like Suns_PSD said, there is so much behind the scenes dealing that goes on that it is basically impossible to know exactly what the dealers are making anymore. I visited 4 dealerships with the exact same build, and I got 4 very different offers. One dealer would not budge from MSRP, and the others were everywhere in between. My advice is to shop around. You may be surprised at the differences in quotes you get. There is a post on here somewhere about a dealership called Laura Buick, that has some unbelievable prices on these trucks, so it is possible to get a really good deal depending on the dealership and the timing of your purchase.
The Eye-Opening Truth About Dealer Invoice Price | www.clarkhoward.com