I used this Firefox add on over the holiday season to buy stuff.
https://addons.mozilla.org/en-US/firefox/addon/the-camelizer-price-history-ch/
The add on tracks the price of items on Best Buy, Amazon, and Newegg. It only goes back a year or so but for the things I was shopping it was very accurate. It can be helpful deciding on when to buy as well as the best price.
I also noticed prices at Dell, Best Buy, Amazon, and Newegg would often change. Usually prices would rise on Friday and drop on Monday. Cheapest prices, like airline tickets, were in the middle of the week with highest prices on the weekend.
EDIT: Once installed the browser has to be restarted. You will see a little Camel in the bottom right of the browser frame. Click on the Camel while viewing a product on Amazon, Newegg or Best Buy. The browser will split in half and show a price history of the item at the three stores.
Later,
Dan