Cheapest priced copy (Mirrodin): $0.99 at Card Kingdom, near mint, their price list as of .
Assert Authority 
Instant · Uncommon · Mirrodin · #30
Affinity for artifacts (This spell costs {1} less to cast for each artifact you control.)
Counter target spell. If that spell is countered this way, exile it instead of putting it into its owner's graveyard.
Legal inCommanderModernLegacyVintage
Not legal inBrawlStandardPioneerPauper
1 printing · $0.99 to $2.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Mirrodin | 30 | Uncommon | $0.99 | $2.49 |