Check DrPraetorious' spell database. It depends very much on the spell, but usually for spells that do area-effect damage it is +1 to AoE and +1 to damage per extra level. Cloud of Death is an exception in that it gets +2 to AoE per extra level. The single-target spells generally scale damage up faster than +1/level.
I think this is the most recent version:
http://www.shrapnelcommunity.com/thr...?Number=559948
-Max