In terms of balance and cost effectiveness, the spells affecting mutplie targets that have either 'base' or 'base plus number of targets' are effected excessively by level - not only does high level give more area of effect, it also spreads the base cost out making each target cheaper.
So, for example, celestial blessing at 4 GB + 4 Gb per target province is very different from net outcome if you can cast it on 2 targets rather than just 1, and still have quite a difference (to those of us who count gold in decimals not tens anyway) between 2 and 3 provinces.
It would be easier to balance spells if the effect as 'x' per unit/province/target than base+'x'.
On the cost:benefit point I vary between hung up (it's just a different way of doing an action) and not fussed (some effects can have huge tactical effect in the right circumstances justifying excessive cost).
From a fluff view I'd rather that a wizard hurl fiery death or summon demons rather than hire mercenaries as otherwise they just stop casting spells and, well, become poor guilders. So I'd look for a balance usually, possibly a slight advantage for magic use to reflect its limitations (need for local source, chance to dispel) or disadvantage for benefits (multiple spells for 1 action, some spells give unexpected tactical advantages or are hard to counter for a non-spellcaster). depending on the spell concerned.
