Cure Disease is essentially replaced at level 32 by Abolish Disease, which removes additional diseases for the same mana cost.
Note that the value, "12% of base Mana," will increase as the Priest levels. However, after factoring in Intellect from gear, this will be approximately equal to 5-10% of your usable mana pool.
The second-tier Discipline talent Absolution can reduce the mana cost of Cure Disease by up to 15%. On the next tier, Mental Agility can reduce the mana cost of instant spells (including Cure Disease) by up to 10%. If one spends the maximum eight talent points in those talents, the mana cost of Cure Disease can be reduced from 12% of Base Mana to 9% of Base Mana.