Amazon has a price guarantee such that if an item you buy goes down in price anytime within 30 days, you are entitled to the difference in price. All you have to do is email them, and they mail you a check. See this link for a more thorough explanation
But that would mean you would have to check back on the prices of things you bought, which is a hassle.
There is a nice website that will track this for you:
So, for each item you purchase just enter in the price, the ISBN (for books) or the ASN (for everything else) and your email address, and they will tell you if the price drops. Pretty handy.