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.
wotc
Dragonlord's Servant can't reduce the colored mana requirement of a Dragon spell you cast.
wotc
If there are any additional costs to cast a Dragon spell, apply those before applying Dragonlord's Servant and any other cost reductions.