Like most Magic: The Gathering cards, the value of Summon: Shiva 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 (Final Fantasy)
Worse still, at the time of writing, you can't trade promo cards in Pokmon TCG Pocket, which means I can't even turn to the community in my hour of need
Card Set(s) -ROMANCE DAWN- [OP01] CARD VIEW OP01-050 | C | CHARACTER Penguin Attribute Strike Block icon 1 Effect [Blocker] (After your opponent declares an attack, you may rest this card to make it the new target of the attack.) [On Play] If you don't have [Shachi], play up to 1 [Shachi] from your hand
For example, if you have 12 cards in your trash and your opponent has 8 cards in theirs, that makes a total of 20 cards, so the play cost is reduced by 8
What were most likely to see is a slow decline in price, regardless of how playable Soulfire actually is