Cheapest priced copy (Battle for Zendikar): $0.35 at Card Kingdom, near mint, their price list as of .
Tajuru Warcaller 
Creature — Elf Warrior Ally · 2/1 · Uncommon · Battle for Zendikar · #195
Rally — Whenever this creature or another Ally you control enters, creatures you control get +2/+2 until end of turn.
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
2 printings · $0.35 to $0.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Battle for Zendikar | 195 | Uncommon | $0.35 | $0.49 |
| The List | BFZ-195 | Uncommon | $0.35 | — |
2 rulings from Wizards of the Coast
- 25 August 2015If a creature with a rally ability enters the battlefield under your control at the same time as other Allies, that ability will trigger once for each of those creatures and once for the creature with the ability itself.
- 25 August 2015When an Ally enters the battlefield under your control, each rally ability of the permanents you control will trigger. You can put them on the stack in any order. The last ability you put on the stack will be the first one to resolve.