The base damage of any instant melee attack that does "weapon damage" is the damage per attack (not damage per second) of the weapon being wielded in the main hand. If your main-hand weapon is listed as "Damage 141-208", for example, your instant attacks will all do a base damage of 141-208 damage points. As a result, a slow weapon will do more damage with an instant attack than a fast weapon with the same rated DPS. To this base damage is added the damage bonus for the maneuver, if any, and a damage bonus based on the wielder's Attack Power.