Cheapest priced copy (Oath of the Gatewatch): $0.49 at Card Kingdom, near mint, their price list as of .
Munda's Vanguard 
Creature — Kor Knight Ally · 3/3 · Rare · Oath of the Gatewatch · #29
Cohort — {T}, Tap an untapped Ally you control: Put a +1/+1 counter on each creature you control.
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
3 printings · $0.49 to $0.59 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Oath of the Gatewatch | 29 | Rare | $0.49 | $0.49 |
| Oath of the Gatewatch Promos | 29 | Rare | — | $0.49 |
| Oath of the Gatewatch Promos | 29s | Rare | — | $0.59 |
One ruling from Wizards of the Coast
- 22 January 2016To activate a cohort ability, the Ally with that ability must have been under your control continuously since the beginning of your most recent turn. Informally, it can’t have “summoning sickness.” However, the other Ally you tap can be one that just came under your control. (Note that tapping the second Ally doesn’t use {T} [the tap symbol].)