Cheapest priced copy (Modern Horizons): $7.49 at Card Kingdom, near mint, their price list as of .
Flusterstorm 
Instant · Rare · Iconic Masters · #55
Counter target instant or sorcery spell unless its controller pays {1}.
Storm (When you cast this spell, copy it for each spell cast before it this turn. You may choose new targets for the copies.)
Legal inCommanderModernLegacyVintage
Not legal inBrawlStandardPioneerPauper
10 printings · $7.49 to $229.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Commander 2011 | 46 | Rare | $10.99 | — |
| Iconic Masters | 55 | Rare | $8.99 | $16.99 |
| Judge Gift Cards 2012 | 2 | Rare | — | $99.99 |
| Mystery Booster 2 | 163 | Rare | $7.99 | $49.99 |
| Modern Horizons | 255 | Rare | $7.49 | — |
| Modern Horizons 3 | 496 | Rare | — | $7.99 |
| The List | CMD-46 | Rare | $14.99 | — |
| Secrets of Strixhaven Mystical Archive | 18 | Mythic | $10.99 | $12.99 |
| Secrets of Strixhaven Mystical Archive | 83 | Mythic | $20.99 | — |
| Secrets of Strixhaven Mystical Archive | 148 | Mythic | — | $229.99 |
5 rulings from Wizards of the Coast
- 8 December 2022A copy of a spell can be countered like any other spell, but it must be countered individually. Countering a spell with storm won't affect the copies.
- 8 December 2022You may choose new targets for any of the copies. You can make different choices for each copy.
- 8 December 2022The triggered ability that creates the copies can itself be countered by anything that can counter a triggered ability. If it is countered, no copies will be put onto the stack.
- 8 December 2022The copies are put directly onto the stack. They aren't cast and won't be counted by other spells with storm cast later in the turn.
- 8 December 2022Spells cast from zones other than a player's hand and spells that were countered are counted by the storm ability.