Actually its an admirable task your about to attempt; but one that i think is wrongheaded.
The problem with magic in Dominions 2 is not that its more powerful than conventional units; after all its supposed to be. The problem is that out of 500 spells, trying to make 475 of them work for you will cost you the game, while 25 of them are so powerful they can win almost irregardless of what your opponent does.
There was a contest in a previous post about an SC competition, but the winning SC apparently won because they used a Mind Control type spell since the SCs weren't marked as Pretenders. One player commented that "you just found a loophole and won the game with it", and that pretty much describes multiplayer Dom2
Find the most absurdly broken spell/summons and run with it, while everyone else is tooling around wondering if flails are better than axes. Not that you can blame them, its a complicated game and it takes time to find these broken abilities, but broken they are.
Now if you want to emphasise conventional troops you need to balance down; if you like magic, than balance up.
In other words, IMO as far as cost and everything, mages are fine. What is not fine are the summons and spells some of them can cast.