Dragonlord's Servant's ability doesn't change the mana cost or mana value of any spell. It changes only the total cost you pay to cast Dragon spells.

Dragonlord's Servant's ability can't reduce the amount of colored mana you pay for a spell. It reduces only the generic mana component of that spell.

Dragonlord's Servant can't reduce the colored mana requirement of a Dragon spell you cast.

If there are any additional costs to cast a Dragon spell, apply those before applying Dragonlord's Servant and any other cost reductions.

The ability can apply to alternative costs to cast a Dragon spell.

To determine the total cost of a spell, start with the mana cost or alternative cost (such as a flashback cost) you're paying, add any cost increases, then apply any cost reductions. The mana value of the spell is determined only by its mana cost, no matter what the total cost to cast the spell was.