The goto deal link takes you straight to the market place page where you will have to login to see the lower price of 400pts. Gold Subscribers only as usual.
Don't forget you can give the demo version a go before pulling the trigger on the full game.
Magic: The Gathering Duels of the Planeswalkers brings the worlds premier trading card game to your living room! Play single and multiplayer campaign modes, test your skills with unique challenges, and engage online opponents via Xbox LIVE. With multiple game modes, an interactive tutorial, leaderboards, and achievements, Duels of the Planeswalkers has something for everyone!