Steam is hosting a sale on Arma 2 and its packages that are required to run the Day Z mod.
The Day Z Mod requires both Arma II and Arma II: Operation Arrowhead, both are included in the Arma 2: Combined Operations bundle but for the best experience (including high detail textures for all models used in the DayZ Mod) purchase Arma II: Complete Collection.
All prices at at 80% discount
Arma 2: Combined Operations - $4.99 (real price $24.99)
Arma 2: Complete Collection - $7.99 (real price $39.99)
The offers end in May 11, 2015.
The Standalone version of DayZ has a long road to go before it is released (Bohemia Interactive stated it will be released during or after mid-2016). Therefore you should not buy it unless you want to be involved in it's development. Meanwhile the Arma 2 Mod version is more stable, has plenty of content and it continues to be supported even after the release of the Standalone version.
You can't go wrong by purchasing this, especially during an 80% sale. Hope to meet you all in-game =)
Here's two random funny video from DayZ standalone =P