Cheapest priced copy (Khans of Tarkir): $0.69 at Card Kingdom, near mint, their price list as of .
Dragon Throne of Tarkir 
Legendary Artifact — Equipment · Rare · The List · #KTK-219
Equipped creature has defender and "{2}, {T}: Other creatures you control gain trample and get +X/+X until end of turn, where X is this creature's power."
Equip {3}
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
3 printings · $0.69 to $1.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Khans of Tarkir | 219 | Rare | $0.69 | $1.99 |
| Khans of Tarkir Promos | 219 | Rare | — | $0.69 |
| The List | KTK-219 | Rare | $0.99 | — |
2 rulings from Wizards of the Coast
- 20 September 2014The value of X is determined as the activated ability resolves. If the equipped creature isn't on the battlefield at that time, use its power the last time it was on the battlefield to determine the value of X.
- 14 July 2017If the equipped creature's power is negative while resolving the activated ability it gains, X is considered to be 0.