It appears Google has changed the rules when it comes to the Google Play store, allowing selected developers to temporarily make apps and games free, as well as show the normal and discounted price in the purchase button.
The changes started showing up a few weeks ago, however Google has apparently now added a support page outlining the changes and how developers can take advantage of them. Paid listings can be discounted through the Dev Console once every thirty days, for up to eight days at a time. Using this discounting method will then show the original, as well as the discounted price in the purchase button.
A major change is that developers can now temporarily make an app free before re-instating the paid price. Previously if an app was made free, the developer was not able to then charge for the app again without relisting it as a new app, and losing all the ratings of the previous app. Apps that are installed when free are then always accessible, just like a paid app.
The support page is now up for any keen developers wanting to check it out, so keep your eye on Google Play for some discounts.