The PriceSCAN.com Web site charts the trends of prices, showing maximum, average, and low. The prices are of products that can be purchased over the Internet, such as computers, software, books, and so on.
For example, here is the price trend for Canon's G2 digital camera:
And for AutoCAD LT. Here you can see the recent price increase (it's now US$899):
This makes buying into like playing the stock market: try to buy at the lowest point before prices turn up.