Cheapest priced copy (Lorwyn): $0.35 at Card Kingdom, near mint, their price list as of .
Whirlpool Whelm 
Instant · Common · Lorwyn · #96
Clash with an opponent, then return target creature to its owner's hand. If you win, you may put that creature on top of its owner's library instead. (Each clashing player reveals the top card of their library, then puts that card on their choice of the top or bottom. A player wins if their card had a greater mana value.)
Legal inCommanderModernLegacyVintagePauper
Not legal inBrawlStandardPioneer
2 printings · $0.35 to $2.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Commander 2011 | 69 | Common | $0.39 | — |
| Lorwyn | 96 | Common | $0.35 | $2.99 |
3 rulings from Wizards of the Coast
- 1 October 2007If you clash with the owner of the targeted creature and win, the owner of the creature decides where to put to card revealed during the clash before you decide whether to put the creature on top of that library. You know the player's decision before you make your decision.
- 1 October 2007You choose the target when you play Whirlpool Whelm, not when you clash.
- 1 October 2007If you don't win the clash, the targeted creature always goes to its owner's hand. If you win the clash, you choose whether the creature goes to its owner's hand or the top of its owner's library.