Developers change the prices of apps all the time to get them into the Top 100 or because of upgrades. Some iPad apps were originally only iphone apps that were free but could be used on the iPad. Gradually these have have been upgraded for the iPad - hence the price changes. As you can imagine it is very difficult to keep up with each and every price change in itunes. The best way of cause is to let someone else do all the work and then get them to send it to you. So the first thing I would suggest is locating a couple of sites that you find useful and then subscribing to their RSS feed.
Lots of websites have been set up as a way of locating appropriate apps. Some of these even have FREE search categories. Others allow you to create wish lists and then they notify you if price drops occur. I like to check the Best of Lists Free Apps every now and again to see if any worthwhile apps have been released. Check these out;
There are even Apps that have been designed to locate FREE apps for the iPad. These Apps are free to purchase and they are regularly updated and make the experience of locating apps even easier. Some of these apps are designed to find apps that are in the categories you designate and then let you know if price drops have occurred. Other are more simply in the tasks they perform.
There are also numerous twitter accounts that specialise in app tracking and some of these you might find useful to follow. I must admit I cut back on these after I had my initial set of apps. I find I do not want my twitter account flooded by just a couple of feeds. Pick the ones that work for you.