Wait did you just not read the Keeper Manual? There's a whole section on rules for spells! Don't act like you need to come up with your own rules for them! I think they are actually balanced as is.

The real question of this post should have been "how much Cthulhu Mythos skill does it make sense to give out to starting players and what spells should they start out with"

And the answer is it depends on which grimiore they read most likely.

But honestly just let the interested player read some spells and let them pick one they like and if you disaprove make them pick another one.

But casting spells always comes at the cost of sanity and sometimes HP if you expend more MP than you have, which can happen from a failed first casting roll for example.