Cheapest priced copy (Alliances): $0.35 at Card Kingdom, near mint, their price list as of .
Mishra's Groundbreaker 
Artifact · Uncommon · Alliances · #123
{T}, Sacrifice this artifact: Target land becomes a 3/3 artifact creature that's still a land. (This effect lasts indefinitely.)
Legal inCommanderLegacyVintage
Not legal inBrawlStandardPioneerModernPauper
1 printing · $0.35 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Alliances | 123 | Uncommon | $0.35 | — |
2 rulings from Wizards of the Coast
- 1 August 2008A noncreature permanent that turns into a creature can attack, and its {T} abilities can be activated, only if its controller has continuously controlled that permanent since the beginning of their most recent turn. It doesn’t matter how long the permanent has been a creature.
- 1 October 2008The ability’s effect has no duration. The affected land will remain an artifact, creature, and land until the game ends, it leaves the battlefield, or some other effect changes its types.