Cheapest priced copy (Commander 2011): $12.99 at Card Kingdom, near mint, their price list as of .
Riddlekeeper 
Creature — Homunculus · 1/4 · Rare · Commander Anthology Volume II · #48
Whenever a creature attacks you or a planeswalker you control, that creature's controller mills two cards.
Legal inCommanderLegacyVintage
Not legal inBrawlStandardPioneerModernPauper
2 printings · $12.99 to $14.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Commander Anthology Volume II | 48 | Rare | $14.99 | — |
| Commander 2011 | 59 | Rare | $12.99 | — |
One ruling from Wizards of the Coast
- 22 September 2011Riddlekeeper doesn’t impose a cost to attack. Creatures can attack you or a planeswalker you control even if their controller has no cards left in their library.