A307

First of all, remember that you can only sneak attack using spells that require an attack roll, and most of these do not allow for saves. The exception to this rule is the Arcane Trickster, who luckily has an FAQ to how their specific feature works:

The Surprise Spells class feature allows the Arcane Trickster to add his sneak attack dice to spells that deal damage that target flat-footed foes. This damage is only applied once per spell. In the case of fireball this means it affects all targets in the area, with each getting a save to halve the damage (including the sneak attack damage). In the case of magic missile, the extra damage is only added once to one missile, chosen by the caster when the spell is cast.
SPECULATION: By this precedent, I would assume that if you are sneak attacking with a spell that requires both an attack roll and a saving throw for half damage, a successful save would half the sneak attack damage as well.