I downloaded the demo version of Magic 2014 on Steam. Now I wanted to look what the full version would cost but when I click on Purchase Full Game this screen appears:
So, where can I purchase the full version?
You are not exactly looking at page to purchase the full version, but rather page for buying the game's DLC (which it doesn't have any), you are in a wrong page if you want to buy the game.
Here is the link to purchase the game itself: http://store.steampowered.com/app/213850/
You can purchase the full version on Magic 2014 Steam Store page through this link.