Cheapest priced copy (Shadows over Innistrad): $0.35 at Card Kingdom, near mint, their price list as of .
Confront the Unknown 
Instant · Common · Shadows over Innistrad · #198
Investigate, then target creature gets +1/+1 until end of turn for each Clue you control. (Create a Clue token. It's an artifact with "{2}, Sacrifice this token: Draw a card.")
Legal inCommanderPioneerModernLegacyVintagePauper
Not legal inBrawlStandard
1 printing · $0.35 to $1.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Shadows over Innistrad | 198 | Common | $0.35 | $1.99 |
5 rulings from Wizards of the Coast
- 8 April 2016You can't cast a spell without choosing legal targets. If all of those targets become illegal, the spell doesn't resolve and you won't investigate.
- 8 April 2016You can't sacrifice a Clue to pay multiple costs. For example, you can't sacrifice a Clue token to activate its own ability and also to activate Alquist Proft, Master Sleuth's ability.
- 8 April 2016No player may take any action in between the two steps of Confront the Unknown, so the target creature will get at least +1/+1.
- 8 April 2016If an effect refers to a Clue, it means any Clue artifact, not just a Clue artifact token. For example, you can sacrifice Wrench to pay for Alquist Proft, Master Sleuth's activated ability.
- 8 April 2016Clue is an artifact type. Even though it appears on some cards with other permanent types, it's never a creature type, a land type, or anything but an artifact type.