I did this last year, with a new page each month in a Moleskin journal (I really like the tactile). Item, date, and (yes, Suz) cost; also, whether or not it was on sale. It was really helpful to see how many items I purchased each month and where the money went. Of course, I am now in a no-buy month, so you see where that led me! I'm going to do it again this year, to help manage my own impulses.
This month, as part of my closet remix, I am trying to document in my WIWs the approximate age of each item I am wearing. My suspicion is that my entire wardrobe has turned over in the last couple of years, but it will help to look at each outfit and figure out how it came together. I don't exactly know what I am going to learn -- how much longevity I have? -- but it's a way to keep me engaged and busy while I am not buying in January.