Cheapest priced copy (Gatecrash): $0.35 at Card Kingdom, near mint, their price list as of .
Boros Keyrune 
Artifact · Uncommon · Gatecrash · #227
{T}: Add {R} or {W}.
{R}{W}: This artifact becomes a 1/1 red and white Soldier artifact creature with double strike until end of turn. (It deals both first-strike and regular combat damage.)
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
2 printings · $0.35 to $0.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| GRN Guild Kit | 96 | Uncommon | $0.39 | — |
| Gatecrash | 227 | Uncommon | $0.35 | $0.49 |
2 rulings from Wizards of the Coast
- 15 April 2013Until the ability that turns the Keyrune into a creature resolves, the Keyrune is colorless.
- 15 April 2013Activating the ability that turns the Keyrune into a creature while it's already a creature will override any effects that set its power and/or toughness to another number, but effects that modify power and/or toughness without directly setting them will still apply.