Cheapest priced copy (Commander Legends: Battle for Baldur's Gate): $0.49 at Card Kingdom, near mint, their price list as of .
Bygone Bishop 
Creature — Spirit Cleric · 2/3 · Rare · Shadows over Innistrad · #8
Flying
Whenever you cast a creature spell with mana value 3 or less, investigate. (Create a Clue token. It's an artifact with "{2}, Sacrifice this token: Draw a card.")
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
4 printings · $0.49 to $1.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Commander Legends: Battle for Baldur's Gate | 689 | Rare | $0.49 | — |
| Shadows over Innistrad Promos | 8s | Rare | — | $1.49 |
| Shadows over Innistrad | 8 | Rare | $0.59 | $1.49 |
| Crimson Vow Commander | 80 | Rare | $0.49 | — |
4 rulings from Wizards of the Coast
- 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 2016Casting a spell for an alternative cost, such as a madness cost, doesn't change its mana value. For example, casting Twins of Maurer Estate (a card with mana cost {4}{B} and madness cost {2}{B}) for its madness cost will not cause Bygone Bishop's ability to trigger.
- 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 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.