Originally Posted by Finkum

I think a non-trivial factor that is being overlooked here is the non-linear way that mana regeneration scales as you add intellect and spirit since the mechanics rework in 2.3. Currently, adding (say) 10 intellect and 10 spirit when at 500int/500spi gives you a greater increase in mana regen than adding 10 int and spi at 250int/250spi.

At 80 (when in raid gear) we can expect to be running around with a fair bit more intellect and spirit than currently, due to base stat increases from 10 more levels, much larger AI/Divine Spirit totals, better enchants and of course higher iLevel gear. This will result in a greater-than-linear increase in mana regeneration (although of course you will need to be using Mage Armor or have 18 points into Arcane to benefit from this), and this might well be one of the reasons why spell costs have increased so dramatically.