Last car I bought, a Subaru Forrester, which is like 25% of cars in Seattle, I knocked $4000, off of msrp by haggling from 28k to 24k, cross shopping across dealers and walking in with a set price I was willing to pay after finding the lowest dealer. Also going in during 2 manufacturer rebates (one cash, one charity match), and at the end of the quarter. 2018s were starting to show up, bought the 2017.
They got real triggered when I said "all you gotta do to put another name up on that trophy board is come down $500 more" (they were writing all of the sales on the window as they happened).
Brought a book to read while they went in to "talk to the manager".
While there is the option to compared to 'no haggle', I can sit there and waste an hour on their BS for $4k. I find it fun and interesting, and I don't buy cars when I have to only when I want to. Helps to have multiple backup transportation methods.
But seriously-- if we're comparing "no haggle" to "haggle" then it doesn't really make sense to include the manufacturer rebates in your total. You'd presumably get those even if dealerships didn't exist.
"Hi, would you like to buy a can of soda for 50 cents? It's warm, because this is a half-ass deal..."