Spell casting cost reduction enchantments can be exploited with game-breaking effects if you use their stacking nature to reach a very high cost reduction or even completely free spells. A good solution would be to disallow stacking of these enchantments, but it would require a major rebalance of spell-costs. A quickfix would be a moderate reduction of the enchantment base magnitude to limit the max cost reduction while keeping single enchanted items useful.
Spellcost reduction enchantments for each school have their base magnitude reduced to 4. (is 6 now)
Robe-specific cost reduction enchantments (with additional magicka regeneration boost) get the same changes applied to their primary effect.