Cheapest priced copy (Oath of the Gatewatch): $0.49 at Card Kingdom, near mint, their price list as of .
Fall of the Titans 
Instant · Rare · Oath of the Gatewatch · #109
Surge {X}{R} (You may cast this spell for its surge cost if you or a teammate has cast another spell this turn.)
Fall of the Titans deals X damage to each of up to two targets.
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
3 printings · $0.49 to $2.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Oath of the Gatewatch | 109 | Rare | $0.49 | $1.99 |
| The List | OGW-109 | Rare | $2.79 | — |
| Oath of the Gatewatch Promos | 109s | Rare | — | $2.99 |
5 rulings from Wizards of the Coast
- 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 2016Fall of the Titan's mana value is based on its mana cost of {X}{X}{R}, even if you're casting it for its surge cost. For example, if you cast Fall of the Titans for its surge cost and choose 4 for X, its mana value will be 9.
- 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.
- 22 January 2016Casting a spell for its surge cost doesn't change its mana cost or its mana value.
- 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.