Cheapest priced copy (Oath of the Gatewatch): $0.35 at Card Kingdom, near mint, their price list as of .
Boulder Salvo 
Sorcery · Common · Oath of the Gatewatch · #102
Surge {1}{R} (You may cast this spell for its surge cost if you or a teammate has cast another spell this turn.)
Boulder Salvo deals 4 damage to target creature.
Legal inCommanderPioneerModernLegacyVintagePauper
Not legal inBrawlStandard
2 printings · $0.35 to $0.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Oath of the Gatewatch | 102 | Common | $0.35 | $0.49 |
| The List | OGW-102 | Common | $0.35 | — |
4 rulings from Wizards of the Coast
- 22 January 2016For some cards, surge represents only an alternative cost, a discount that applies if you or a teammate has cast another spell this turn. Other cards, like Reckless Bushwhacker, have additional abilities or effects if you paid the surge cost to cast the spell.
- 22 January 2016If an instant or sorcery spell cast for its surge cost is copied, the copy is also considered to have had its surge cost paid.
- 22 January 2016Casting a spell for its surge cost doesn’t change its mana cost or its mana value.
- 22 January 2016The other spell that you or a teammate cast can be one that’s resolved, one that was countered, or (for instants with surge) one that’s still on the stack.