Damage is going to be very stat- and gear-dependent. But with a successful backstab+sneak combo (with Guerilla), you should be getting a 4x damage multiplier. Couple that with the fact that daggers hit twice, and you end up dealing 8x stated weapon damage (plus any bonuses from abilities/gear) with a single attack.
For reference, my level 7 shadowblade deals around 180 damage per hit (so 360 damage total per attack) to same-level enemies with a backstab+sneak using a dagger with a base damage of somewhere around 40-60, iirc. Also my guy has nowhere near an optimal build in terms of stat distribution and gear. So I have no doubt you could see higher numbers at the same level.
