Sadly, the second cards whereabouts are completely unknown, with only one card ever surfacing on the market
If the spell that's copied has an X whose value was determined as it was cast, the copy will have the same value of X
Like most Pokmon cards, the value of Jigglypuff depends on a few consistent factors: its rarity, the printing and finish (regular or foil), its physical condition, and the set it was printed in (XY)
Since it's a mana cheap instant, you can do this in response to targeted or sweeping removal to get cards, as a sacrifice or it can send something to your graveyard about to get exiled by Swords to Plowshares, or to get value out of a chump blocker about to die anyway
Grimgrin becomes threatening once token production begins