I copied this from another blog by Eddie Wong. Never knew about this myself.
The process for claiming a refund is easy, here are the steps:
Launch iTunes
Click on iTunes Store
Login to your iTunes account, if you’re already logged in click on your email in the upper right corner of iTunes
Click on “Purchase History”
Select the app that you want a refund for
Click on “Report a Problem”
Fill out the form detailing the problem with the app purchase and why you want a refund
Submit the problem report and wait for a response from Apple
The amount of time you have to wait varies, but it’s usually a pretty quick claims process.
Nice one Eddie.