Cheapest priced copy (Commander 2020): $0.49 at Card Kingdom, near mint, their price list as of .
Swarm Intelligence 
Enchantment · Rare · Commander 2020 · #124
Whenever you cast an instant or sorcery spell, you may copy that spell. You may choose new targets for the copy.
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
4 printings · $0.49 to $2.29 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Commander 2020 | 124 | Rare | $0.49 | — |
| Commander 2021 | 130 | Rare | $0.49 | — |
| Hour of Devastation | 50 | Rare | $0.79 | $1.99 |
| Hour of Devastation Promos | 50s | Rare | — | $2.29 |
8 rulings from Wizards of the Coast
- 14 July 2017If the spell has damage divided as it was cast (like Chandra’s Pyrohelix), the division can’t be changed (although the targets receiving that damage still can).
- 14 July 2017If the spell that’s copied has an X whose value was determined as it was cast (like Torment of Hailfire does), the copy will have the same value of X.
- 14 July 2017If the spell that’s copied is modal (that is, it says “Choose one —” or the like), the copy will have the same mode. A different mode can’t be chosen.
- 14 July 2017Swarm Intelligence can copy any instant or sorcery spell, not just one with targets.
- 14 July 2017Swarm Intelligence can copy the spell even if it’s countered before Swarm Intelligence’s triggered ability resolves.
- 14 July 2017The copy is created on the stack, so it’s not “cast.” Abilities that trigger when a player casts a spell (such as Swarm Intelligence’s own ability) won’t trigger.
- 14 July 2017The controller of a copy can’t choose to pay any alternative or additional costs for the copy. However, effects based on any alternative or additional costs that were paid for the original spell are copied as though those same costs were paid for the copy.
- 14 July 2017The copy will have the same targets as the spell it’s copying unless you choose new ones. You may change any number of the targets, including all of them or none of them. If, for one of the targets, you can’t choose a new legal target, then it remains unchanged (even if the current target is illegal).