Cheapest priced copy (Dragons of Tarkir): $0.49 at Card Kingdom, near mint, their price list as of .
Illusory Gains 
Enchantment — Aura · Rare · Dragons of Tarkir · #59
Enchant creature
You control enchanted creature.
Whenever a creature an opponent controls enters, attach this Aura to that creature.
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
1 printing · $0.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Dragons of Tarkir | 59 | Rare | $0.49 | $0.49 |
4 rulings from Wizards of the Coast
- 25 February 2015The last ability of Illusory Gains doesn't target the new creature. It will become attached to that creature even if that creature has hexproof or shroud.
- 25 February 2015Illusory Gains's last ability is mandatory. You can't choose to have Illusory Gains not move.
- 25 February 2015If Illusory Gains can't legally enchant the creature that enters the battlefield (perhaps because it has protection from blue), it remains where it is. You retain control of the creature it's currently enchanting.
- 25 February 2015If multiple creatures enter the battlefield under an opponent's control at the same time, Illusory Gains will trigger for each of them. You may put these abilities on the stack in any order. As each resolves, you'll briefly gain control of the associated creature. The last one to resolve determines which one you'll ultimately control.