Cheapest priced copy (Betrayers of Kamigawa): $0.35 at Card Kingdom, near mint, their price list as of .
Horobi's Whisper 
Instant — Arcane · Common · Betrayers of Kamigawa · #70
If you control a Swamp, destroy target nonblack creature.
Splice onto Arcane—Exile four cards from your graveyard. (As you cast an Arcane spell, you may reveal this card from your hand and pay its splice cost. If you do, add this card's effects to that spell.)
Legal inCommanderModernLegacyVintagePauper
Not legal inBrawlStandardPioneer
2 printings · $0.35 to $0.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Betrayers of Kamigawa | 70 | Common | $0.35 | $0.49 |
| Modern Masters | 88 | Uncommon | $0.35 | $0.49 |
5 rulings from Wizards of the Coast
- 1 February 2005Horobi’s Whisper can target a creature even if you don’t control a Swamp, but the spell does nothing.
- 7 June 2013A card with a splice ability can’t be spliced onto itself because the spell is on the stack (and not in your hand) when you reveal the cards you want to splice onto it.
- 7 June 2013You choose all targets for the spell after revealing cards you want to splice, including any targets required by the text of any of those cards. You may choose a different target for each instance of the word “target” on the resulting spell.
- 7 June 2013You reveal all cards you intend to splice at the same time. Each individual card can be spliced only once onto any one spell.
- 7 June 2013If all of the spell’s targets are illegal when the spell tries to resolve, it won’t resolve and none of its effects will happen.