Copypasted from UESP:
Cost = base cost × skill multiplier × perk multiplier × equipment multiplier × dual cast multiplier
Skill multiplier = 1 − (skill/400)^0.65
In other words, the latter. Apply the 41% reduction first, then the 25% enchantment.
Copypasted from UESP:
Cost = base cost × skill multiplier × perk multiplier × equipment multiplier × dual cast multiplier
Skill multiplier = 1 − (skill/400)^0.65
In other words, the latter. Apply the 41% reduction first, then the 25% enchantment.