Cheapest priced copy (Duskmourn: House of Horror Commander): $0.59 at Card Kingdom, near mint, their price list as of .
Massacre Girl 
Legendary Creature — Human Assassin · 4/4 · Rare · War of the Spark · #99
Menace
When Massacre Girl enters, each other creature gets -1/-1 until end of turn. Whenever a creature dies this turn, each creature other than Massacre Girl gets -1/-1 until end of turn.
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
10 printings · $0.59 to $39.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Duskmourn: House of Horror Commander | 146 | Rare | $0.59 | — |
| Marvel Universe (Elektra, Deadly Assassin) | 67 | Mythic | $0.99 | $3.49 |
| Outlaws of Thunder Junction Commander | 140 | Rare | $0.69 | — |
| War of the Spark Promos | 99s | Rare | — | $2.99 |
| Ravnica Remastered | 83 | Rare | $0.79 | $2.49 |
| Ravnica Remastered | 328 | Rare | $2.29 | $3.49 |
| Ravnica Remastered | 328z | Rare | — | — |
| Ravnica Remastered | 425 | Rare | $7.49 | $17.99 |
| Secret Lair Drop | 1233 | Rare | $34.99 | $39.99 |
| War of the Spark | 99 | Rare | $0.79 | $1.99 |
5 rulings from Wizards of the Coast
- 3 May 2019Once Massacre Girl's enters-the-battlefield ability has triggered, it doesn't matter whether Massacre Girl remains on the battlefield. The delayed triggered ability will be created as the enters-the-battlefield ability resolves.
- 3 May 2019If more than one creature dies at once, Massacre Girl's delayed triggered ability triggers that many times.
- 3 May 2019Creatures that die while Massacre Girl's triggered ability is still on the stack won't cause its delayed triggered ability to trigger since that delayed triggered ability hasn't been created yet.
- 3 May 2019Massacre Girl's enters-the-battlefield triggered ability and its delayed triggered ability each affect only creatures on the battlefield at the time that those abilities resolve. Creatures that enter the battlefield or become creatures later in the turn won't get -1/-1 unless the delayed triggered ability triggers and resolves again later.
- 3 May 2019A creature with 0 toughness doesn't die immediately. Rather, the creature dies the next time a player would receive priority. This means that any creatures whose toughness becomes 0 as Massacre Girl's enters-the-battlefield ability resolves will remain on the battlefield until just after the delayed triggered ability has been created.