Cheapest priced copy (Dragons of Tarkir): $0.35 at Card Kingdom, near mint, their price list as of .
Gate Smasher 
Artifact — Equipment · Uncommon · Dragons of Tarkir · #239
This Equipment can be attached only to a creature with toughness 4 or greater.
Equipped creature gets +3/+0 and has trample.
Equip {3}
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
1 printing · $0.35 to $0.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Dragons of Tarkir | 239 | Uncommon | $0.35 | $0.49 |
2 rulings from Wizards of the Coast
- 25 February 2015If Gate Smasher is attached to a creature whose toughness falls below 4, Gate Smasher will become unattached as a state-based action.
- 25 February 2015Gate Smasher's equip ability can target any creature. However, if that creature's toughness is 3 or less as the equip ability resolves, Gate Smasher won't become attached to it.