Cheapest priced copy (Duel Decks: Mind vs. Might): $0.49 at Card Kingdom, near mint, their price list as of .
Sage-Eye Avengers 
Creature — Djinn Monk · 4/5 · Rare · Fate Reforged · #50
Prowess (Whenever you cast a noncreature spell, this creature gets +1/+1 until end of turn.)
Whenever this creature attacks, you may return target creature to its owner's hand if its power is less than this creature's power.
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
4 printings · $0.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Duel Decks: Mind vs. Might | 8 | Rare | $0.49 | — |
| Fate Reforged | 50 | Rare | $0.49 | $0.49 |
| Fate Reforged Promos | 50 | Rare | — | $0.49 |
| Fate Reforged Promos | 50s | Rare | — | $0.49 |
4 rulings from Wizards of the Coast
- 24 November 2014Prowess goes on the stack on top of the spell that caused it to trigger. It will resolve before that spell.
- 24 November 2014Prowess triggers only once for any spell, even if that spell has multiple types.
- 24 November 2014Once it triggers, prowess isn’t connected to the spell that caused it to trigger. If that spell is countered, prowess will still resolve.
- 24 November 2014Any spell you cast that doesn’t have the type creature will cause prowess to trigger. If a spell has multiple types, and one of those types is creature (such as an artifact creature), casting it won’t cause prowess to trigger. Playing a land also won’t cause prowess to trigger.