Cheapest priced copy (Commander 2020): $0.35 at Card Kingdom, near mint, their price list as of .
Temur Charm 
Instant · Uncommon · Khans of Tarkir · #208
Choose one —
• Target creature you control gets +1/+1 until end of turn. It fights target creature you don't control.
• Counter target spell unless its controller pays {3}.
• Creatures with power 3 or less can't block this turn.
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
2 printings · $0.35 to $0.79 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Commander 2020 | 230 | Uncommon | $0.35 | — |
| Khans of Tarkir | 208 | Uncommon | $0.35 | $0.79 |
4 rulings from Wizards of the Coast
- 20 September 2014If the third mode was chosen, creatures with power 3 or less that enter the battlefield after Temur Charm resolves also can’t block. A creature whose power was 4 or greater when Temur Charm resolved but is 3 or less as the declare blockers step begins also can’t block.
- 20 September 2014If the first mode was chosen and either target is an illegal target when Temur Charm tries to resolve, neither creature will deal or be dealt damage.
- 20 September 2014If the first mode was chosen and just the creature you control is an illegal target when Temur Charm tries to resolve, it won’t get +1/+1 until end of turn. If the creature you control is a legal target but the creature you don’t control isn’t, the creature you control will get +1/+1 until end of turn.
- 20 September 2014You can’t cast Temur Charm choosing the first mode unless you target both a creature you control and a creature you don’t control.