You are spot on with your research. I do very similar when purchasing, but don't go as far as keeping a spread sheet. When I went to buy my last big purchase, I was drawn in by one dealership who had what I thought I wanted for $25,995. They said they were negotiable if I came down in person. I went down with $26k cash for an out the door deal. After the test drive, they gave me an out the door price of $28,995:shocked:. Needless to say, I showed them the money and then walked out.
2 weeks later I went to look at a newer model that was listed for $31,995. They also said they were negotiable and gave me an out the door price of $29,995. I went to walk away and they asked what it would take to sell that day. I offered $28k cash. They said no, but then agreed if I would finance for atleast 3 months (I found out that they get $3k kick back from the bank). I agreed, but they owed me a couple of things (convertible cover and the extra key fob). After taking delivery of the car and financing $27.5k, they then wanted to charge me for the extra key fob ($350). I said no and went to their main dealership. The owner finally gave me the key fob the next week, and I paid the car off that following week, so the sales team missed out on the extra $3k from the bank.