AppShopper is free and available as an app on your device or online from their website. You can create a wish list of apps that you would like to purchase in the future. You will receive a notification if the price changes on the app. If you don't think you will log into the site or don't want to install the app on your device, you can also request to receive email notifications when any of your wish list apps go on sale.
Make sure you check out the App Activity to see the price history as well as update history. You can see how often it goes on sale and predict when it might go on sale again. Below you can see that the Dr. Seuss Band was on sale from February-April 2012. If it ever goes on sale again, it would likely be part of the Seuss birthday celebrations around March 2nd.
Dr. Seuss Band is only 99 cents. This is a great price for an app, but I haven't attempted spending school money on apps yet. The process described by our tech department sounded very complicated and I've avoided it, so far, by downloading free apps.
If you missed my post of 110 Free Music Apps, follow this link!