Skarner's ult mana cost.

Seeing the new buffs (nerfs?) to Sona's mana cost on her ultimate I was just curious if Skarner has a reason for his ult scaling in mana cost? It currently gains 25 mana in cost per rank (100/125/150). I guess you could argue that it's more effective or reliable than sona's ultimate but then I would just say it should just be 125 or 150 mana cost from rank one.