Cheapest priced copy (Theros): $0.35 at Card Kingdom, near mint, their price list as of .
Boulderfall 
Instant · Common · Theros · #115
Boulderfall deals 5 damage divided as you choose among any number of targets.
Legal inCommanderPioneerModernLegacyVintagePauper
Not legal inBrawlStandard
1 printing · $0.35 to $0.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Theros | 115 | Common | $0.35 | $0.49 |
3 rulings from Wizards of the Coast
- 15 September 2013If some but not all of Boulderfall's targets become illegal, you can't change the division of damage. Damage that would have been dealt to illegal targets simply isn't dealt.
- 15 September 2013If an effect creates a copy of Boulderfall, the division of damage and number of targets can't be changed. The effect that creates the copy may allow you to change targets, however.
- 15 September 2013You divide the damage as you cast the spell, not as it resolves. Each target must be assigned at least 1 damage.