Cheapest priced copy (Tarkir: Dragonstorm Commander): $0.49 at Card Kingdom, near mint, their price list as of .
Colfenor's Urn 
Artifact · Rare · Tarkir: Dragonstorm Commander · #315
Whenever a creature with toughness 4 or greater is put into your graveyard from the battlefield, you may exile it.
At the beginning of the end step, if three or more cards have been exiled with this artifact, sacrifice it. If you do, return those cards to the battlefield under their owner's control.
Legal inCommanderModernLegacyVintage
Not legal inBrawlStandardPioneerPauper
2 printings · $0.49 to $13.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Lorwyn | 254 | Rare | $0.69 | $13.99 |
| Tarkir: Dragonstorm Commander | 315 | Rare | $0.49 | — |
3 rulings from Wizards of the Coast
- 1 October 2007The first ability triggers when a token creature with toughness 4 or greater is put into your graveyard. However, that token will cease to exist before Colfenor’s Plans can exile it.
- 1 October 2007The second ability forces you to sacrifice Colfenor’s Urn. If the ability triggers and the Urn isn’t sacrificed (because it left the battlefield or changed controllers, for example), the exiled cards won’t be returned to the battlefield.
- 1 October 2007The second ability checks how many creatures have been exiled with Colfenor’s Urn over the course of the entire game.