Cheapest priced copy (Guilds of Ravnica): $0.35 at Card Kingdom, near mint, their price list as of .
Take Heart 
Instant · Common · Guilds of Ravnica · #28
Target creature gets +2/+2 until end of turn. You gain 1 life for each attacking creature you control.
Legal inCommanderPioneerModernLegacyVintagePauper
Not legal inBrawlStandard
2 printings · $0.35 to $0.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Guilds of Ravnica | 28 | Common | $0.35 | $0.49 |
| Jumpstart | 134 | Common | $0.35 | — |
2 rulings from Wizards of the Coast
- 5 October 2018If the target creature becomes an illegal target for Take Heart, the spell doesn’t resolve. You don’t gain life.
- 5 October 2018You can cast Take Heart even if you control no attacking creatures, perhaps because it’s not even your turn. The target creature gets +2/+2 even though you don’t gain life.