That's exactly the way I handle it. I do my research, figure out a price for my trade, figure out what I'd like them to come down to on their vehicle so that the prices are fair for them and me, estimate LEGITIMATE fees (license, title, sales tax) and say this is the out the door price I'd like, I'm not here to haggle. Work the numbers how you want on my trade, on your vehicle.
I did that with my recent Palisade purchase at my local dealership. They tried their best to get me to come up from the price I gave them, first, by countering $800 higher, then, the next day, they said they would agree to my out the door price, but by "out the door" they didn't mean it would include taxes. (LOL -- you're a car dealer and you don't know what "out the door" means?!?) So in the next text they said they would "split the taxes" with me, making my price $1100 higher or something like that, in other words, even higher than their first counter of $800. I'm not sure if they're that stupid or if they thought I was that stupid. Anyway, I let a few more days pass and figured I should just show up at their dealership with the title for my trade and my payment, and I came up $250 from the initial price I gave them, just to throw them a bone, because you got to make them feel like they "won" something, for their ego. I said we either do this deal now or I'm going to XXX dealership 50 miles away and presenting them with the same offer (fortunately that dealer had an identical Palisade Limited, except it was $500 more with the $250 price increase and a couple options like mudguards). It wasn't 1 minute later they came back and shook my hand and said "congrats on your new car, thanks for being patient with us." In other words, "thanks for not leaving even though we did our best to take advantage of you."
This was actually the first time I felt better about purchasing a car. I felt "better," not "good," because how can you feel good when someone is your best friend one minute, and then when it comes to talking price they turn into total jack$ss and lie to your face? For me, car buying is one of those experiences that makes you want to keep your car a long, long time, so that you can delay going through the whole process again.