Cheapest priced copy (Khans of Tarkir): $0.35 at Card Kingdom, near mint, their price list as of .
Valley Dasher 
Creature — Human Berserker · 2/2 · Common · Khans of Tarkir · #125
Haste
This creature attacks each combat if able.
Legal inCommanderPioneerModernLegacyVintagePauper
Not legal inBrawlStandard
2 printings · $0.35 to $0.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Khans of Tarkir | 125 | Common | $0.35 | $0.49 |
| The List | KTK-125 | Common | $0.35 | — |
3 rulings from Wizards of the Coast
- 20 September 2014If, during your declare attackers step, Valley Dasher is tapped or is affected by a spell or ability that says it can't attack, then it doesn't attack. If there's a cost associated with having Valley Dasher attack, you're not forced to pay that cost, so it doesn't have to attack in that case either.
- 20 September 2014If Valley Dasher enters the battlefield before the combat phase, it will attack that turn if able. If it enters the battlefield after combat, it won't attack that turn and will usually be available to block on the following turn.
- 20 September 2014You still choose which player or planeswalker Valley Dasher attacks.