For Destruction magic, the amount of XP earned is based both on the magicka cost of the spell and on damage done to the target. Since your damage output is variable based on your difficulty level, you will get less XP at difficulties higher than Adept(however, you will not get bonus XP for Novice or Apprentice).
If you can enchant a set of gear that makes Destruction Spells free to cast ( helps if the set is free cast Restoration as well), you can cast costly spells as much as you want. Some of the best for raising the skill are actually the rune spells. They don't do a ton of damage(50 damage and the Augent ELEMENT perks don't modify them), but they cost almost as much to cast as expert spells.
Since they don't do as much damage, you can hit targets more for more XP( you can even heal them and yourself for Restoration XP). Runes are apprentice spells, so they are available from the start of the game from any court mage without the need to raise the skill at all before they become available for purchase(unlike adept or expert level spells which are unavailable for purchase until your Destruction skill is at 40 or 65 respectively).