You may not want to know how much you've spent on apps since you picked up your first iPhone. After all, most of the cost in owning an iPhone is in the monthly data plan, right? App Store Expense Monitor will calculate your app total automatically.
The program is free and runs on Mac OS X 10.5 or later. Simply run the program and it scans your iTunes library to determine what apps you've already bought. The software then checks the current price of each application and outputs a total cost.