I found an addon yesterday, called "Market Watcher", which monitors Auction House prices of items you set and then draws graphs so you can see how the prices have changed. From the site:

Market Watcher is an addon that scans the auction house and records data for specified items and assists in daily auction house shopping in general. When the addon is set to record, a price history can be displayed to assist in determining a value for items of interest.

What makes Market Watcher unique from other auction house addons is that it records multiple scans instead of just the most recent. This obviously uses a great deal of memory, so only scans of items specified by the user will be recorded. The saving of multiple scans allows for a more accurate determination of value, and allows the addon to deduce if some auctions were actually sold, so you can see which auctions are actually selling as opposed to basing a value off the prices of auctions that may not be selling at all.

While it's a great addon, it does something I don't like: it doesn't let you open your trade skill windows while the Auction House window is open - the reason for this is because (normally) opening a trade skill window closes the Auction House window, but if you're using the Auctioneer addon there's an option to "protect auction house window" to allow you to have both open. To remove this "protection"/"feature" of Market Watcher, open "MarketWatcher.lua" in your "Interface\AddOns\MarketWatcher" folder, and find "UIParent:UnregisterEvent("TRADE_SKILL_SHOW");" (line 255 in the 5th Aug 2009 version) and put two minus signs (--) in front, to comment it out.

You'll need to restart WoW, or do a "/reloadui" to apply these changes.