Cheapest priced copy (Mirrodin): $0.35 at Card Kingdom, near mint, their price list as of .
Thought Prison 
Artifact · Uncommon · Mirrodin · #261
Imprint — When this artifact enters, you may have target player reveal their hand. If you do, choose a nonland card from it and exile that card.
Whenever a player casts a spell that shares a color or mana value with the exiled card, this artifact deals 2 damage to that player.
Legal inCommanderModernLegacyVintage
Not legal inBrawlStandardPioneerPauper
1 printing · $0.35 to $0.79 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Mirrodin | 261 | Uncommon | $0.35 | $0.79 |
One ruling from Wizards of the Coast
- 1 December 2004The second ability triggers only once per spell. If the spell shares both a color and a mana value with the imprinted card, Thought Prison deals 2 damage, not 4.