Cheapest priced copy (Zendikar): $0.49 at Card Kingdom, near mint, their price list as of .
Electropotence 
Enchantment · Rare · Zendikar · #122
Whenever a creature you control enters, you may pay {2}{R}. If you do, that creature deals damage equal to its power to any target.
Legal inCommanderModernLegacyVintage
Not legal inBrawlStandardPioneerPauper
1 printing · $0.49 to $0.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Zendikar | 122 | Rare | $0.49 | $0.99 |
4 rulings from Wizards of the Coast
- 1 October 2009Electropotence is the source of the ability, but the creature is the source of the damage. The ability couldn't target a creature with protection from red, for example. It could target a creature with protection from creatures, but all the damage would be prevented. Since damage is dealt by the creature, lifelink, deathtouch, and wither are taken into account, even if the creature has left the battlefield by the time it deals damage.
- 1 October 2009Once you decide to pay {2}{R}, it's too late for anyone to respond (such as by activating a regeneration ability the targeted creature has).
- 1 October 2009If you pay {2}{R}, the creature that entered deals damage equal to its current power to the targeted permanent or player. If it's no longer on the battlefield, its last known existence on the battlefield is checked to determine its power.
- 1 October 2009You target a permanent or player when the ability triggers. You decide whether or not to pay {2}{R} as the ability resolves.