Here are a few tools that I use every time I want to purchase an item online
Compares the prices of many of the popular online retailers. You can specify a price range and apply a sorting algorithm to the results to find what you're looking for.
This site provides price comparison directly to other retailers but also provides graphs of price trends over time.
Finally camelegg provides great technology price comparison over a few major retailers (amazon, bestbuy, newegg). You can see price trending over time and even setup notifications to get an email or a tweet when a price hits the range in which you are willing to shell out the money for it.