FOILCheapest priced copy (War of the Spark): $0.49 at Card Kingdom, near mint, their price list as of .
Vivien's Arkbow 
Legendary Artifact · Rare · War of the Spark Promos · #181s
{X}, {T}, Discard a card: Look at the top X cards of your library. You may put a creature card with mana value X or less from among them onto the battlefield. Put the rest on the bottom of your library in a random order.
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
2 printings · $0.49 to $0.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| War of the Spark Promos | 181s | Rare | — | $0.69 |
| War of the Spark | 181 | Rare | $0.49 | $0.99 |
One ruling from Wizards of the Coast
- 3 May 2019If a card in your library has {X} in its mana cost, X is considered to be 0.