If you buy from Amazon, you should be using Amazon Price Watch. Here’s how it works… You just put in the URL to what you purchased, along with your email address, and they notify you when the price for the item drops.
Why is this useful, you ask? Because Amazon has a price guarantee that, for 30 days, if the price drops, they’ll happily refund you the difference, for as many times as they drop the price. I’ve probably gotten about $50 in refunds using this service this past month to monitor my new camera equipment.