Cheapest priced copy (Mirage): $0.35 at Card Kingdom, near mint, their price list as of .
Crypt Cobra 
Creature — Snake · 3/3 · Uncommon · Mirage · #114
Whenever this creature attacks and isn't blocked, defending player gets a poison counter. (A player with ten or more poison counters loses the game.)
Legal inCommanderLegacyVintage
Not legal inBrawlStandardPioneerModernPauper
1 printing · $0.35 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Mirage | 114 | Uncommon | $0.35 | — |
2 rulings from Wizards of the Coast
- 4 October 2004The player gets a poison counter as a triggered ability on not blocking the Cobra. Killing the Cobra after declaring blockers or preventing its damage will not prevent the poison counter.
- 15 April 2013An ability that triggers when something “attacks and isn’t blocked” triggers in the declare blockers step after blockers are declared if (1) that creature is attacking and (2) no creatures are declared to block it. It will trigger even if that creature was put onto the battlefield attacking rather than having been declared as an attacker in the declare attackers step.