Firebolt - Mana cost is..a tad bit high.
Fireball - 1 dual-casted fireball and it's almost all of your mana - even with 100 destruction and 300 Magicka.
Incinerate - 300 magicka, I can't dual cast this.
Fire Storm - I can't cast this yet.
I'm lv 22, put every point into magicka, pretty much only leveld Destruction - and I ..quite simply.. can't kill a dragon in under ~20 minutes due to magicka. I play on master, and it takes me roughly 15 magicka bars to kill a Frost Dragon. That...seems a bit ridiculous. I'm not playing enchanting, so I only have -12% destruction cost reduction.
Enchanting is REQUIRED to play destruction effectivly - This is terrible. This is why Pure Destro mages are so rare. Their damage is Plenty - their magicka efficiency is not. Single casting Incinerates takes up roughly a third of my magicka bar - and 1shots most anythign I come across outside of bosses/dragons. But 3 casts later and I either wait for a day, or hide around the corner til I have moar magicka. Of my 6 characters now - this is by FAR my least favorite.
Take my conjurer for example - I'm lv 26, I do 129 damage per bow shot, roughly 400 damage on my first shot, and in a few more levels I'll even be paralyzing things I shoot. Couple that with my 2 Awesome pets, tanky and burney, who both do about as much dps as my destruction does - individually. It's no wonder why no one wants to play Destro. :/
And oh - Just for fun, Once you hit -100% you can just charge Master Spark(whatever it's called - reminds me of master spark from touhou =P) on anything and they just MELT to you. But being mana-free, You just spam it endlessly, with no repercussions. People can't be finding that enjoyable =/. I want SOME difficulty in my game, where as destruction literally goes from Near-impossible, to near-invincible who can melt everything in seconds. Seriously - to anyone playing destruction, shed your self-enchanted gear for just 1 dragon fight - see how long it takes ya. It's not cool.