Specifically, you can grab Halo: Spartan Assault for just $1.99 in either the Windows Phone Store or the Windows Store. That's an amazing price, because the game offers 30 total missions, which is up from the 25 that were offered when it first launched. The game is a top-down twin-stick shooter that is designed to fill in the gap in the storyline between Halo 3 and Halo 4.
There are also in-app purchases, but none are required. They offer limited weapon upgrades to help you get through a challenging level, but nothing permanent. And, the game is available in 13 languages, so it's a good deal for international users as well. It is a pretty hefty game though, so you'll need a higher-end smartphone to run it.