Cheapest priced copy (Magic 2012): $0.35 at Card Kingdom, near mint, their price list as of .
Slaughter Cry 
Instant · Common · Zendikar · #149
Target creature gets +3/+0 and gains first strike until end of turn. (It deals combat damage before creatures without first strike.)
Legal inCommanderModernLegacyVintagePauper
Not legal inBrawlStandardPioneer
2 printings · $0.35 to $0.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Magic 2012 | 155 | Common | $0.35 | $0.49 |
| Zendikar | 149 | Common | $0.35 | $0.49 |
One ruling from Wizards of the Coast
- 1 October 2009If a creature doesn’t have first strike, granting it first strike after combat damage has been dealt in the first combat damage step won’t prevent it from dealing combat damage. It will still assign and deal its combat damage in the second combat damage step.