Amazon has ArmA2: Combined Operations for $19.99 (33% off). Definitely not the cheapest it’s been but it’s been harder to get a discount since the DayZ mod came out.
I would still wait and see if Steam has this as a daily deal at some point during their sale though. Unless you want it DRM free because this will NOT activate on Steam.
Personally I’m waiting for this game to go on sale on Steam also. Hoping for $10 or so.