Cheapest priced copy (Dragons of Tarkir): $0.35 at Card Kingdom, near mint, their price list as of .
Keeper of the Lens 
Artifact Creature — Golem · 1/2 · Common · Dragons of Tarkir · #240
You may look at face-down creatures you don't control any time.
Legal inCommanderPioneerModernLegacyVintagePauper
Not legal inBrawlStandard
1 printing · $0.35 to $0.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Dragons of Tarkir | 240 | Common | $0.35 | $0.49 |
3 rulings from Wizards of the Coast
- 25 February 2015Keeper of the Lens doesn’t stop your opponents from looking at face-down creatures they control.
- 25 February 2015Keeper of the Lens allows you to look at face-down creatures you don’t control whenever you want to, even if you don’t have priority. This action doesn’t use the stack.
- 25 February 2015Keeper of the Lens doesn’t let you look at other players’ face-down spells while they’re on the stack.