Depends on what you do. If you max out enchanting, you can get your gear to reduce magicka costs for two different spells of casting. I have a battlemage that wears dragonplate armor with a beige hood, and my restoration and destruction spells don't cost anything.
Another solution is using the atronoch stone plus the atronoch perk in the alteration tree. The character I created with that is essentially an anit-mage, able to withstand sustained enemy magic attacks by absorbing them to replenish her magicka while hitting them with chain lightning and incinerate.
Additionally, with the Aetherium Crown from the quest added in the Dawnguard DLC, you can have two standing stone abilities at the same time. I put the Apprentice stone on it, so the magicka regeneration hit you take from the Atronoch stone is effectively negated, in addition to the increased magicka absorbtion you would get from fire attacks thanks to the weakness to fire that the Apprentice stone gives. It's a really effective combo for tackling enemy magick users.
The best scenario I had was when I was in a cave with about 5 lightning mages, all hitting me with lightning spells. I never took any damage, and I basically had infinite magicka.