The DA means that the hammer is cocked and released by a single motion (i.e. the trigger being pulled). Single action (SA) means that the hammer is cocked prior to the trigger being pulled, and the trigger is used to release the hammer and thus fire the weapon. With the 590DA, they wanted to duplicate the trigger action of most police sidearms, which are DA. Thus, the shotgun's trigger now cocks and fires the weapon, assuming that it is chambered. The result is a heavier trigger pull and the relative illusion of enhanced safety and weapon system commonality. As for the going price, that's not horrible.
