How about this: If the spells are going to get more expensive at higher levels, make the spells much more powerful as they go along.
So, if you're using 2% Int for Level 1, for 2 Mana, use 4% Int for Level 2 for 3 Mana.
That means that Mana increases by +1, while damage increases by a factor of 2, AND is dependent on your Intelligence.
You can switch how some spells use damage, but make them increase by more than the cost of Mana.
Example:
% = Percentage of Intelligence
n = Additional Damage
M = Mana Cost
Use (% + n) / M and see that it increases at each level.
(2 + 0) / 2 = 1
(4 + 0) / 3 = 1.333
(6 + 0) / 4 = 1.5
...
(2L) / (L + 1)
Whereas, right now, you have a spell like this:
(10 + 1) / 2 = 5.5
(10 + 2) / 3 = 4
(10 + 3) / 4 = 3.25
...
(L + 10) / (L + 1)
I think if you even do something like this, it'll be better:
(10 + 1) / 2 = 5.5
(11 + 2) / 3 = 4.333
(12 + 3) / 4 = 3.75
(13 + 4) / 5 = 3.4
(14 + 5) / 6 = 3.166
(10L + L - 1 + L) / (L + 1)
It becomes less efficient still, but it allows for much more powerful spells as well. It's just if you only increase by a set number for each additional Mana Point put in, instead of a percentile increase, it's not worth it.
This also makes spell levels more useful, since you can try and do less damage to less powerful creatures, or more powerful spells to bosses.
I think you should also allow xp at a decreased rate, something like: Spell Level / Max Spell Level * xp gained, since they are using a less powerful version of that spell.