Spell Cost

I have noticed that as you level certain magic classes, the mana cost will increase to compensate the spells damage increase. I completely understand that and think it’s a great mechanic to the game. The only issue I have had, as a pure mage, the drain mana and drain mana touch spells are bugged. For a pure mage, this spell is key to keeping up your mana points, but I realize that as I level my mind and personality and intelligence points, the cost of these spells are way too high compared to how much is actually drained. I am now sitting at 100 in mind magic with 75 in intelligence and 40 in personality. When I cast drain mana or drain mana touch, they both cost about 100 plus mana points but will only give me 70 mana points... I’m spending more points to use the spell than I am receiving back. What am I missing with this spell? Am I doing my leveling up wrong or is the spell just not properly balanced?