Cheapest priced copy (Foundations Jumpstart): $0.35 at Card Kingdom, near mint, their price list as of .
Tenacity 
Instant · Uncommon · Shadows over Innistrad · #42
Creatures you control get +1/+1 and gain lifelink until end of turn. Untap those creatures. (Damage dealt by those creatures also causes you to gain that much life.)
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
3 printings · $0.35 to $0.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Foundations Jumpstart | 104 | Uncommon | $0.35 | — |
| The List | SOI-42 | Uncommon | $0.35 | — |
| Shadows over Innistrad | 42 | Uncommon | $0.35 | $0.49 |
2 rulings from Wizards of the Coast
- 8 April 2016The set of creatures affected by Tenacity is determined as the spell resolves. Creatures you begin to control later in the turn won't get +1/+1 or gain lifelink.
- 8 April 2016Multiple instances of lifelink on the same creature are redundant.