I had installed Windows on my MacBook Air after wiping its HDD and then decided to have OS X reinstalled. One thing I noticed is that the office software that had previously showed up in the dock wasn't installed. When I look up Pages on the Appstore it shows a 20€ price tag. How can that be?
If it was associated with your Apple ID before the OS X reinstallation the reason is that you are haven't logged into the Mac App Store with your Apple ID.
Go to 'Store > Sign In…' and enter your Apple ID credentials. After that you will see "Download" instead of the price for Pages.