Bohemia Interactive are having a 25% off sale for everything in their store, this includes Arma II, Arma II Operation Arrowhead, Arma II Combined Ops. You can also choose from a Steam or Non-Steam version. Personally I had purchased this last fortnight on steam for $30 and it's definitely worth it if you want to try out DayZ!
Arma II Combined Operations Steam Version:
https://store.bistudio.com/arma2-combined-operations-steam-v…
Arma II Combined Operations Non-Steam Version:
https://store.bistudio.com/arma2-combined-operations
Chances are that Arma II will be cheaper during the up coming Steam Sale.