That seems to depend largely on the dev or publisher. Criterion have released several good free updates for Burnout Paradise along side their premium offerings. And there's also been free map packs for Gears of War and Halo 3. However on the whole most DLC content needs to be paid for, and despite it being a couple or more years ago, we are still seeing "Horse Armour" style offerings from some companies. For instance Dead Space and Far Cry 2's offerings have been rather poor for the money asked.
At the end of the day though, I, like any other consumer will vote with my wallet as to what I think is worth paying for. As for possible DLC content I'd like to see for ArmA2; I primarily played OFP and ArmA for the co-op experience, so more good quality co-op missions are something I'd gladly stump up money for.