Cheapest priced copy (Dragons of Tarkir): $0.35 at Card Kingdom, near mint, their price list as of .
Sabertooth Outrider 
Creature — Human Warrior · 4/2 · Common · Dragons of Tarkir · #152
Trample
Formidable — Whenever this creature attacks, if creatures you control have total power 8 or greater, this creature gains first strike until end of turn.
Legal inCommanderPioneerModernLegacyVintagePauper
Not legal inBrawlStandard
1 printing · $0.35 to $0.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Dragons of Tarkir | 152 | Common | $0.35 | $0.49 |
3 rulings from Wizards of the Coast
- 25 February 2015If you control a creature with power less than 0, use its actual power when calculating the total power of creatures you control. For example, if you control three creatures with powers 4, 5, and -2, the total power of creatures you control is 7.
- 25 February 2015Other formidable abilities are triggered abilities with an “intervening ‘if’” clause. Such abilities check the total power of creatures you control twice: once at the appropriate time to see if the ability will trigger, and again as the ability tries to resolve. If, at that time, the total power of creatures you control is no longer 8 or greater, the ability will have no effect.
- 25 February 2015Some formidable abilities are activated abilities that require creatures you control to have total power 8 or greater. Once you activate these abilities, it doesn’t matter what happens to the total power of creatures you control.