Hmmm... instant 2-damage AoE is cost of 3
and 2:air (T-storm) and instant 3-damage AoE is cost of 7
and 5
(RoF). This card suffers from two turns in which intervention can be done to prevent the AoE damage. To me, that seems like cost between them is about right for a high-end damage effect that is delayed/preventable. So, 4
and 3
-- however since the intervention requires the opponent to kill their own creature to stop it so 5/4 might be right.
Oh wait - does this damage your own creatures as well? If so, then a -1 cost reduction to both upgraded and normal might be fair...