Cheapest priced copy (Worldwake): $0.35 at Card Kingdom, near mint, their price list as of .
Permafrost Trap 
Instant — Trap · Uncommon · Worldwake · #34
If an opponent had a green creature enter the battlefield under their control this turn, you may pay {U} rather than pay this spell's mana cost.
Tap up to two target creatures. Those creatures don't untap during their controller's next untap step.
Legal inCommanderModernLegacyVintage
Not legal inBrawlStandardPioneerPauper
1 printing · $0.35 to $0.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Worldwake | 34 | Uncommon | $0.35 | $0.49 |
3 rulings from Wizards of the Coast
- 1 March 2010If a targeted creature is untapped at the time its controller's next untap step begins, the second part of the ability has no effect on it.
- 1 March 2010If an affected creature changes controllers before its previous controller's next untap step, this ability will prevent it from untapping during its new controller's next untap step.
- 1 March 2010You may target any two (or fewer) creatures. It's okay if any of them are already tapped. If you cast Permafrost Trap for {U}, you don't have to target the green creature that entered under an opponent's control.