[Endoperez]

It varies wildly. I checked Dr Praetorious' spell guide, and it seems to put most damage spells at +1 or +2 damage/extra level. Some exceptions (stun seems to be +2 to +5/level, heal and healing touch are is +10/level, touch spells often give like +5 dam/level), but nothing major.

Hmm, Smite Demon is +5/level with 0 base, while banishment is +2/level with 3 base... Priest communions with holy levels above 5 start to favor Smite Demon against demon thugs, because it deals so much damage, but that's not important.