App Shopper allows you to save Apps to a wish list which will then send a notification when prices drop. Hand if you don’t think to check the free apps at the store. There is also a companion website that you can visit and browse apps listed that might not be on your wish list.. This app works for iPhone as well.
There are ads on the free version. If they bug you they can be removed with an upgrade for a small fee.
Get it at the App store: https://itunes.apple.com/us/app/appshopper/id387037496?mt=8
Visit their website: http://appshopper.com