Check out these websites
Check out slickdeals.com, or camelcamelcamel.com to get an idea of available prices.
Slickdeals allows you to set up email alerts in case anyone finds a deal on whatever you are interested in. Also, they have a rating system where members give a deal a thumbs up or a thumbs down based on whether they think the price is a good deal or not.
Camelcamelcamel gives you a graph that shows the recent price changes for the item on Amazon. This way you can get an idea of whether the current price is good, the best, or might go down in the future.