Cheapest priced copy (Commander 2015): $0.35 at Card Kingdom, near mint, their price list as of .
Cobra Trap 
Instant — Trap · Uncommon · Zendikar · #160
If a noncreature permanent under your control was destroyed this turn by a spell or ability an opponent controlled, you may pay {G} rather than pay this spell's mana cost.
Create four 1/1 green Snake creature tokens.
Legal inCommanderModernLegacyVintage
Not legal inBrawlStandardPioneerPauper
3 printings · $0.35 to $0.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Commander 2015 | 180 | Uncommon | $0.35 | — |
| Commander 2011 | 146 | Uncommon | $0.35 | — |
| Zendikar | 160 | Uncommon | $0.35 | $0.49 |
4 rulings from Wizards of the Coast
- 1 October 2009Casting a Trap by paying its alternative cost doesn’t change its mana cost or mana value. The only difference is the cost you actually pay.
- 1 October 2009A spell or ability destroys a permanent only if that spell or ability specifically contains the word “destroy” in its text. If a spell or ability an opponent controls exiles a noncreature permanent you control, causes you to sacrifice a noncreature permanent, removes all loyalty counters from a planeswalker you control, or causes an ability you control to trigger (and then that ability destroys a permanent you control), Cobra Trap’s alternative cost condition hasn’t been met.
- 1 October 2009Effects that increase or reduce the cost to cast a Trap will apply to whichever cost you chose to pay.
- 1 October 2009You may ignore a Trap’s alternative cost condition and simply cast it for its normal mana cost. This is true even if its alternative cost condition has been met.