I would go one step further actually. What you are saying, if I understood correctly, is that each time the ward blocks a spell yout magicka should decrease in a fix percentage.
Not a fix percentage, a fix percentage of the damage received. You are hit by Firebolt (30 damage), you lose 30 magicka if you block with a Novice ward, u are hit by Fireball by 140 u lose 140 magicka. If you use an Expert ward (based on my example) you will lose 15/70 magicka. So if you block master spells with a novice ward your magicka will be gone in a flash, not so with an master ward.
In my opinion, it would be better if ward's cost was completely variable and proportional to the strength of the spell being blocked (by doing this, expert-level spells would cost much more magicka to block than novice-level spells and so on...)
That's exactly what I meant. The levels (Novice-Master) just define how fast your magicka is drained with hits.
However, and while I agree with this approach, for me overhauling ward's cost is a secondary goal. In my opinion, wards svck because 1) you cannot cast them just before being hit and 2) they are way too easy to break (40-80 points of damage in a short period of time is nothing).
1) If the dual cast spells are longer maybe the single ones speed can be reduced so that it equals the speed of a warrior raising a shield.
Now, If a variable cost system similar to what I proposed was to be implemented, making wards not svck anymore would be so easy as to implement the following mechanics:
1.Wards only consume magicka when blocking spells.
2.Wards will only break if the caster runs out of magicka.
Is the same thing I said so, I like it

1) I havent tested wards much (because of how much they svck), but I think they block any damage (spell, melee, ranged), right?
2) Yes
The only problem I see is that the Novice spells will be useless and wont scale (like now). There should be some scaling/use for novice spells. Something like...
Novice: Attack drains 90% of damage as magicka, heal 10% of damage
Apprentice: Attack drains 80% of damage as magicka, heal 7% of damage
Adept: Attack drains 70% of damage as magicka, heal 5% of damage
Expert: Attack drains 60% of damage as magicka, heal 3% of damage
Master: Attack drains 50% of damage as magicka, heal 0% of damage
That way novice spells could still be useful at higher levels.