I don't keep track as in write them down and study them but if there is a book on my mind or if I want to kick myself over the missed opportunity, I kind of remember or if I am tracking a book, I look up the historical sales data.
For instance, I was walking around NYCC one year with Jaybuck shortly before the Fox-Disney deal was announced. I was looking at two books, both priced around $4500. There was an FF 1 in a 4.5 and an ASM 1 in a 4.5 with a dust shadow. I really wanted to upgrade my ASM1 at the time so I passed on the FF1. I wasn't too pleased with the dust shadow and said let me walk around for a bit. When I went back the FF1 was gone but the ASM1's dust shadow bothered me. By the time of the next month's round of auctions from CC, Heritage, CLink, MCS and eBay the ASM 1 popped... I tried chasing it but other things came up and by the time I said I might as well just get an FF1 for the heck of it, the Disney deal was announced and that popped. Such is life.
PetefromPetesBasement has been trying to get me to see the importance of Franklin Richards and Annihilus for a couple of years but I just couldn't get myself to pull the trigger on an FF Annual 6. When 8.0's were around $200 I said, "This is not a $200 book" but I took note of the price. When it was $300, I said, "This is not a $300 book" but I took note of the price and the trend.
Finally, a couple of weeks ago I bought the 7.0 that was on Metro's website for $425, which somehow was under what everyone one else wants for the book. (They actually made a counteroffer and I took them up on it).
But... I keep track of the prices of books I've passed on but I am still somewhat interested in. I think we all do.