Spells with cascade care about their own mana value, but discover spells define X themselves
1 box = 4 packs
The first one released was Shivan Dragon at MagicCon Philadelphia, the second one was Giant Growth at MagicCon Minneapolis, the third was Lord of the Pit at MagicCon Barcelona, the fourth one was Merfolk of the Pearl Trident at Gen Con, and the fifth one was Swords to Plowshares at MagicCon Vegas
(Equipment, Auras you control, and counters are modifications.) The value of X needs to be determined both when the ability triggers (so you can choose a target) and again when the ability resolves (to check if that target is still legal)
Certainly making it a pleasant surprise for anyone who manages to pull this rare card
While this seems like a good pre-emptive countermeasure to azetsu222s problem, there are a couple of reasons its actually not