Cheapest priced copy (Explorers of Ixalan): $0.35 at Card Kingdom, near mint, their price list as of .
Urge to Feed 
Instant · Uncommon · Worldwake · #70
Target creature gets -3/-3 until end of turn. You may tap any number of untapped Vampire creatures you control. If you do, put a +1/+1 counter on each of those Vampires.
Legal inCommanderModernLegacyVintage
Not legal inBrawlStandardPioneerPauper
5 printings · $0.35 to $2.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Duel Decks: Sorin vs. Tibalt | 22 | Uncommon | $0.39 | — |
| Explorers of Ixalan | 21 | Uncommon | $0.35 | — |
| Foundations Jumpstart | 502 | Uncommon | $0.35 | — |
| Crimson Vow Commander | 139 | Uncommon | $0.35 | — |
| Worldwake | 70 | Uncommon | $0.35 | $2.99 |
3 rulings from Wizards of the Coast
- 1 March 2010If you cast Urge to Feed targeting an untapped Vampire creature you control with toughness 3, its toughness will be reduced to 0 by the -3/-3 effect. However, state-based actions aren't checked until the spell has finished resolving. You may then tap that Vampire and put a +1/+1 counter on it, raising its toughness to 1. It will remain on the battlefield.
- 1 March 2010You don't choose which untapped Vampire creatures you control to tap (if any) until Urge to Feed resolves.
- 1 March 2010If the targeted creature is an illegal target by the time Urge to Feed resolves, the entire spell doesn't resolve. You can't tap any Vampires.