A couple of weeks ago I asked the following question on Twitter: "dear twitterverse, is there an easy way to determine the total amount of money you've spent on iphone apps?"
I received no responses, which I thought was quite odd, but just struck it up to there being no simple (or even difficult, but still non-manual) way to get the number I was after. I was wrong. The App Store Expense Monitor gives you the number immediately, and even lets you view a complete list of your purchased apps (including, obviously, their respective costs).
Turns out I've spent $110 on iPhone apps, which is less than I would have guessed.