I'm not sure if my game is bugged or what but I have in total 75% reduction to Destruction and the perks, yet I can only cast 20 Incinerates with my 500 Magicka pool. Most enemies take 8 or so to kill them. This just canceled my Destruction Mage build, I will have to use conjuration. It seems Beth never thought about using high level destruction.

without 90 or higher cost reduction, the cost for the expert level spells is just outrageous, i dont know why they veered away from the cost reduction we got in oblivion......where a spell that cost 450 at lelve 30 costs only 40 magicka at skill 95 or higher.

only? I find expert spells very low cost-effective, I'm level 54 right now and I have Ice Spike (apprentice) hot-key'd.
Apprentice is good for prolonged battles, adept is good for groups, expert is good for 1v1.

You need to either A. find better reduction gear, or B. make your own. You really need 90-100% reduction past level 40. My recommendation late game would be master robes(22% reduction + magicka regen), and a few items with 22-25% reduction. Giving you a total in the 90-97% area. Not as game breaking as 100%, but enough to keep you from running dry in seconds.

You should also be dual casting, uses more magic but you will kill enemies in 3-4 hits instead of 8.