Cheapest priced copy (Strixhaven: School of Mages): $0.35 at Card Kingdom, near mint, their price list as of .
Blood Researcher 
Creature — Vampire Druid · 2/2 · Common · Strixhaven: School of Mages · #166
Menace (This creature can't be blocked except by two or more creatures.)
Whenever you gain life, put a +1/+1 counter on this creature.
Legal inCommanderPioneerModernLegacyVintagePauper
Not legal inBrawlStandard
1 printing · $0.35 to $0.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Strixhaven: School of Mages | 166 | Common | $0.35 | $0.49 |
4 rulings from Wizards of the Coast
- 16 April 2021If you gain an amount of life “for each” of something, that life is gained as one event and the ability will trigger only once.
- 16 April 2021An ability that triggers “whenever you gain life” triggers just once for each life gain event, no matter how much life you gain.
- 16 April 2021If you gain life at the same time Blood Researcher is dealt lethal damage, it dies before its ability can put a +1/+1 counter on it.
- 16 April 2021Each creature with lifelink dealing combat damage causes a separate life gain event. For example, if two creatures you control with lifelink deal combat damage at the same time, a “whenever you gain life” ability will trigger twice. However, if a single creature you control with lifelink deals combat damage to multiple creatures, players, and/or planeswalkers at the same time (perhaps because it has trample or was blocked by more than one creature), the ability will trigger only once.