The mySalesAgent 2 is an OS X App which will keep an eye on the prices of Apps you want to buy once they reach a targeted price.
It is available for free at http://www.schneppi-software.de (English and German Website).
It offers a clean interface in which the user can search for iPhone/Universal Apps, iPad Apps, and Mac OS X Apps, available on Apples Stores. Just search for Apps which you want to purchase when they reach your targeted price. Set your price and forget about it. The mySalesAgent 2 will inform you via OS X Notification Center Messages and/or via Email when an item reaches the targeted price.