Cheapest priced copy (Legends): $0.49 at Card Kingdom, near mint, their price list as of .
Rust 
Instant · Common · Legends · #203
Counter target activated ability from an artifact source. (Mana abilities can't be targeted.)
Legal inCommanderLegacyVintagePauper
Not legal inBrawlStandardPioneerModern
1 printing · $0.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Legends | 203 | Common | $0.49 | — |
2 rulings from Wizards of the Coast
- 4 October 2004Can’t be used on mana abilities because they don’t use the stack. They’ve already resolved by the time you get priority.
- 8 June 2016Activated abilities contain a colon. They’re generally written “[Cost]: [Effect].” Some keywords are activated abilities and will have colons in their reminder text.