Cheapest priced copy (Aether Revolt): $0.59 at Card Kingdom, near mint, their price list as of .
Ajani's Aid 
Enchantment · Rare · Aether Revolt · #188
When this enchantment enters, you may search your library and/or graveyard for a card named Ajani, Valiant Protector, reveal it, and put it into your hand. If you search your library this way, shuffle.
Sacrifice this enchantment: Prevent all combat damage a creature of your choice would deal this turn.
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
1 printing · $0.59 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Aether Revolt | 188 | Rare | $0.59 | — |
2 rulings from Wizards of the Coast
- 9 February 2017The last ability of Ajani’s Aid doesn’t target anything. You choose a source of damage as the ability resolves.
- 9 February 2017You can activate the last ability even if there is no creature about to deal combat damage, or even no creature at all. No damage will be prevented, but it will enable your revolt abilities.