The list keeps changing, but if you type 'worlds most expensive apps 2012' in a search engine you will find several different lists, including a Youtube video that shows you what you will get for your money. You can also find specific examples, such as the following:

The most expensive app on iTunes and Google Play will cost you just under £600. However, unless you work in the steel and scrap metal recycling industry you probably won't be needing it. It's the Alchemist SMS and it's intended to help you get your raw materials more cheaply.

At the same price is the BarMax CA and CarMax NY - again, a bit of a niche market here. This one is to help law students prepare for their Bar exams.

As of February 2012, certain iTunes apps cost up to $1000 each, which is actually about £640, so they're even more expensive.