Cheapest priced copy (Dominaria United): $0.35 at Card Kingdom, near mint, their price list as of .
Twinferno 
Instant · Uncommon · Dominaria United · #149
Choose one —
• When you cast your next instant or sorcery spell this turn, copy that spell. You may choose new targets for the copy.
• Target creature you control gains double strike until end of turn. (It deals both first-strike and regular combat damage.)
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
3 printings · $0.35 to $0.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Dominaria United | 149 | Uncommon | $0.35 | $0.99 |
| Foundations Jumpstart | 606 | Uncommon | $0.35 | — |
| The List | DMU-149 | Uncommon | $0.35 | — |
4 rulings from Wizards of the Coast
- 9 September 2022If the spell that's copied has damage divided as it was cast, the division can't be changed (although the targets receiving that damage still can). The same is true of spells that distribute counters.
- 9 September 2022You can't choose to pay any additional costs for the copy created by Twinferno's delayed triggered ability. However, effects based on any additional costs that were paid for the original spell are copied as though those same costs were paid for the copy too.
- 9 September 2022If the spell that's copied is modal (that is, it says "Choose one —" or the like), the copy created by Twinferno's delayed triggered ability will have the same mode or modes. You can't choose different ones.
- 9 September 2022The copies that Twinferno's ability creates are created on the stack, so they're not "cast." Abilities that trigger when a player casts a spell won't trigger.