Cheapest priced copy (Final Fantasy Commander): $0.79 at Card Kingdom, near mint, their price list as of .
Torrential Gearhulk 
Artifact Creature — Construct · 5/6 · Mythic · Kaladesh · #67
Flash
When this creature enters, you may cast target instant card from your graveyard without paying its mana cost. If that spell would be put into your graveyard, exile it instead.
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
6 printings · $0.79 to $54.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Commander Masters | 128 | Rare | $0.99 | $2.49 |
| Commander Masters | 501 | Rare | — | — |
| Final Fantasy Commander | 272 | Rare | $0.79 | $1.99 |
| Kaladesh | 67 | Mythic | $0.99 | $2.99 |
| Kaladesh Inventions | 2 | Special | — | $54.99 |
| Kaladesh Promos | 67s | Mythic | — | $5.99 |
3 rulings from Wizards of the Coast
- 20 September 2016If you cast the card, you do so as part of the resolution of Torrential Gearhulk's triggered ability. You can't wait to cast it later in the turn. Timing restrictions (such as "Cast [this card] only during combat") still apply.
- 20 September 2016If you cast a card "without paying its mana cost," you can't choose to cast it for any alternative costs, such as emerge costs. You can, however, pay additional costs. If the card has any mandatory additional costs, such as that of Incendiary Sabotage, you must pay those to cast the card.
- 20 September 2016If the card has {X} in its mana cost, you must choose 0 as the value of X when casting it without paying its mana cost.