To some extent I try and ignore what Im paying for a car, or what Im getting in part exchange allowance. The thing I always focus on is the "cost to change" i.e. the difference.
Whenever Ive bought a car Ive;
1. Checked what the absolute maximum is I could get for my car if I sold it privately.
2. Searched for the best price for the new car, regardless of where it might come from.
3. 2-1= your target "cost to change".
Ive walked out of dealers before, many times, when they refuse to budge.
Plus, always make them feel that youve got another option lined up elsewhere..."if you dont do this deal, youll make no profit at all as Ill go buy the car from (insert other dealers name). So I can do that, or you can sell me a car, make a bit less profit on the sale, and then lots more when I bring it back for servicing...?"
Its worked very well for me so far...