Cheapest priced copy (Lorwyn Eclipsed Commander): $0.49 at Card Kingdom, near mint, their price list as of .
Slithermuse 
Creature — Elemental · 3/3 · Rare · Morningtide · #50
When this creature leaves the battlefield, choose an opponent. If that player has more cards in hand than you, draw cards equal to the difference.
Evoke {3}{U} (You may cast this spell for its evoke cost. If you do, it's sacrificed when it enters.)
Legal inCommanderModernLegacyVintage
Not legal inBrawlStandardPioneerPauper
2 printings · $0.49 to $9.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Lorwyn Eclipsed Commander | 69 | Rare | $0.49 | — |
| Morningtide | 50 | Rare | $0.69 | $9.99 |
One ruling from Wizards of the Coast
- 1 April 2008You choose an opponent when the ability resolves. Once you determine how many more cards than you that player has, that number is locked in as the amount you'll draw.