Fall 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.

For 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.

The 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.

If an instant or sorcery spell cast for its surge cost is copied, the copy is also considered to have had its surge cost paid.

Casting a spell for its surge cost doesn't change its mana cost or its mana value.