Cheapest priced copy (Dominaria): $0.35 at Card Kingdom, near mint, their price list as of .
Serra Disciple 
Creature — Bird Cleric · 1/1 · Common · Dominaria · #34
Flying, first strike
Whenever you cast a historic spell, this creature gets +1/+1 until end of turn. (Artifacts, legendaries, and Sagas are historic.)
Legal inCommanderPioneerModernLegacyVintagePauper
Not legal inBrawlStandard
2 printings · $0.35 to $0.49 at Card Kingdom
4 rulings from Wizards of the Coast
- 27 April 2018An ability that triggers “whenever you cast a historic spell” doesn’t trigger if a historic card is put onto the battlefield without being cast.
- 27 April 2018A card, spell, or permanent is historic if it has the legendary supertype, the artifact card type, or the Saga subtype. Having two of those qualities doesn’t make an object more historic than another or provide an additional bonus—an object either is historic or it isn’t.
- 27 April 2018Lands are never cast, so abilities that trigger “whenever you cast a historic spell” won’t trigger if you play a legendary land. They also won’t trigger if a card on the battlefield transforms into a legendary land, as the Ixalan and Rivals of Ixalan double-faced cards do.
- 27 April 2018Some abilities trigger “whenever you cast a historic spell.” Such an ability resolves before the spell that caused it to trigger. It resolves even if that spell is countered.