Mishra's Self-Replicator is a rare Artifact Creature — Assembly-Worker card from Dominaria (DOM), collector number 223. Current market price: $0.19 (foil $0.64), TCGplayer market prices via Scryfall.
Whenever you cast a historic spell, you may pay {1}. If you do, create a token that's a copy of this creature. (Artifacts, legendaries, and Sagas are historic.)