It's a nice idea as long as magicka drains accordingly to the amount of time the beneficial spell affects you.
For example: if a restore health 10 pts spell costs 30 magicka, casting it for 5 seconds (50 pts) will cost 150 magicka.
I agree with the point of your post, not with the math. We have 2 spells 1 with a magnitude of 10 and duration of 1 (cost: 30) and the other is 50 magnitude and 5 seconds (In Oblivion that would mean healing 50 points a second for 5 seconds so 250 points of health) for a cost of 150.
If you break down the first spell the Base cost is 3 magicka for per one magnitude, with every additional second costing an additional amount. So 10 by 1 = 30, and 10 by 2 = 60.
So that means that the second spell would be 15 points for the base spell, then multiply it times the number of seconds. You are looking at a cost of 750 if my math was right. Anyone wanna check that?