Cheapest priced copy (Fallen Empires): $0.35 at Card Kingdom, near mint, their price list as of .
Delif's Cone 
Artifact · Common · Fallen Empires · #84
{T}, Sacrifice this artifact: This turn, when target creature you control attacks and isn't blocked, you may gain life equal to its power. If you do, it assigns no combat damage this turn.
Legal inCommanderLegacyVintagePauper
Not legal inBrawlStandardPioneerModern
1 printing · $0.35 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Fallen Empires | 84 | Common | $0.35 | — |
3 rulings from Wizards of the Coast
- 4 October 2004If you use the ability after blockers are declared, it won’t trigger at all. So you want to use it before blockers are declared.
- 4 October 2004This card has an activated ability that creates a triggered ability which watches the target creature.
- 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.