I think for completing a Monster Arena quest you recieve 99 Dark Matter. But the task is collecting 5 of each capturable monster (if I'm wrong someone correct me please.)
But for the effort of gathering all of the Dark Matter you'd be better off aquiring all of the Celestial Weapons and powering them up. If nothing else the tasks are all different so it'd be less monotonous then fighting boss monsters over and over and over and over and over ad nauseum.
