Cheapest priced copy (Murders at Karlov Manor Commander): $0.35 at Card Kingdom, near mint, their price list as of .
Ongoing Investigation 
Enchantment · Uncommon · Shadows over Innistrad · #77
Whenever one or more creatures you control deal combat damage to a player, investigate. (Create a Clue token. It's an artifact with "{2}, Sacrifice this token: Draw a card.")
{1}{G}, Exile a creature card from your graveyard: Investigate. You gain 2 life.
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
2 printings · $0.35 to $0.79 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Murders at Karlov Manor Commander | 114 | Uncommon | $0.35 | — |
| Shadows over Innistrad | 77 | Uncommon | $0.35 | $0.79Out of stock |
4 rulings from Wizards of the Coast
- 8 April 2016In a Two-Headed Giant game, if you control more than one creature that can attack, you may have them attack different opponents so that Ongoing Investigation's first ability triggers twice.
- 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.
- 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 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.