Problem
At the moment the ultimate way for spell research is (if water or earth magic):
Research maximum Spelllevel -> Research Earthquake or Flood -> exterminate all enemies
This result of the current (low) Price of Spelllevels seems strange.
If you research in an more typical way, researching all available spells before progressing a spell level, then casters with much spellbooks will obtain the more powerfull spells with great delay compared to casters possessing only one spellbook.
Possible Solution
Make Spellevel Research much more costly [about the price of 5-8 Spells (numbers to be tweaked) of the respective level]. Subtract half the price of learned spells (of a given level) of the price to increase the next spellevel.
So if you reserach 10-16 (numbers to be tweaked) Spells per level the spelllevel increase is for free. If you however decide not to learn any spells, or possess few spellbooks you pay the full price. This way researching spells along the way (while doubling the total time) will be more efficent then beelining to end game spells.