I still think we should have the mp cost reduced during combo's but i have found out ways to make the mp consumption better.

Thunder chain goes like this: Cast thunder > wait till combo timer reaches 2 then cast thundara( by this time you should have full mp if you waited to combo till the timer was down to 2 seconds) > Then use pars for half mp cast and mana gain, this should cut the spell to about 30% of its cost on boss mobs.