Jchobot posted... best i can do is fortify destruction and with a grand gem and corpus encahnter and 5/5 enchanter its only 16%!
It takes four items to make destruction spells free. If your enchanting skill is maxed, use fortify destruction on four different items (and whatever other enchants you want with the double enchant perk). I enchanted a ring, an amulet, my dragonscale helmet and my dragonscale armor with fortify destruction and a mixture of other enchants.
Enchant each item (ring, necklace, helmet, armor) with fortify destruction and grand soul gem. Right now, being at 16% that would be a total reduction of 64%. You need to get your enchanting up to at least 80, and then use a fortify enchanting to get it up the rest of the way to 100. Or just get enchanting to 100. Once its at 100, each item you enchant should be "Destruction Spells cost 25% less" so added together its 100% less.
I recommend getting the double enchant perk in the tree so you can add two enchantments on each item. This gives you the ability to cast two trees of magic (destruction and conjuration for me) for free.
Oh, and make you you have the arcane enchanter perk for more fun enchanting old weapons.