Cheapest priced copy (Mystery Booster 2): $14.99 at Card Kingdom, near mint, their price list as of .
Stifle 
Instant · Rare · Mystery Booster 2 · #173
Counter target activated or triggered ability. (Mana abilities can't be targeted.)
Legal inCommanderLegacyVintage
Not legal inBrawlStandardPioneerModernPauper
7 printings · $14.99 to $1299.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Conspiracy | 108 | Rare | $15.99 | $39.99 |
| Judge Gift Cards 2009 | 3 | Rare | — | $94.99Out of stock |
| Mystery Booster 2 | 173 | Rare | $14.99 | $54.99 |
| Amonkhet Invocations | 18 | Special | — | $84.99 |
| Scourge | 52 | Rare | $37.99 | $1299.99 |
| Secret Lair Drop (Beholder's Sleep Ray) | 1787 | Rare | $21.99 | $23.99 |
| World Championship Decks 2004 | gn52sb | Rare | $14.99 | — |
3 rulings from Wizards of the Coast
- 4 October 2004It can target delayed triggered abilities. For example, a card that says "Whenever a player cycles a card, you may exile target creature. If you do, return that creature to the battlefield at the beginning of the end step." triggers when a player cycles a card and creates a delayed trigger that happens at the beginning of the end step. You can choose to target the "at the beginning of the end step" trigger when it is placed on the stack at end of turn.
- 4 October 2004An activated ability has a "Cost: Effect" format. Look for the colon. A triggered ability starts with "when", "whenever", or "at".
- 4 October 2004Turn-based actions and special actions like the normal card draw, combat damage, or turning a face-down creature face up can't be targeted.