Economics doesn't really apply to the world of Golarion if you apply the Rules As Written. Just look a the equipment list!
All of the prices have fixed values. That doesn't happen in a real economy. It makes sense for a game, but it doesn't really happen. In real life prices fluctuate in response to supply and demand.
If you have a kingdom that uses Scrolls of Fireball by the ton, the price isn't going to stay at 375gp. If the supply of Scrolls of Fireball can't be changed, then the price is going to go up. The price could possibly hit the point where it wouldn't be feasible to use them on the battlefield.
If the supply of Scrolls of Fireball can be increased, it becomes much less clear about how the price will be affected, but you can be sure that more of them will be bought.
With that said, there is an economics term I would like to introduce to the debate.
"Barriers To Entry"
http://en.wikipedia.org/wiki/Barriers_to_entry
Basically, there are certain things you have to do if you want to get into a business. Sometimes these things are cheap, sometimes they are very expensive.
If you want to be a farmer, all you need are some seeds and some land to plant them in. This is easy to do (especially if you live in a feudal society where the local lord will let you farm his land in exchange for 70% of the harvest).
If you want to be a doctor, a lawyer or a king, you're going to have to put in a little more work before you can update your business cards.
Doctors and lawyers have to go through years of school, and then pass a series of tests. Becoming a king generally involves either founding a kingdom (no small task) or creating a vacancy in another kingdom at the exact point in time that everyone in the kingdom (that is, everyone whose opinion matters) thinks that you should be the new king (also, not an easy thing to do).
"What does this have to do with magical warfare?"
Simply put, Barriers to Entry don't exist in Pathfinder for PCs.
If you want to make a Wizard? *Poof!* Your PC is Joe the wizard.
If you want to make a Fighter? *Poof!* Joe the PC is now a fighter.
Decided that you really want to play a Barbarian? *Poof!* Meet Joe the Barbarian.
This makes sense, as it makes it so you can play the character you want.
(Also, a slight nod *is* made to Barriers to Entry, as reflected by each class having a different starting wealth range)
What does this have to do with this thread?
NPC's would have to pass the Barriers to Entry if they would like to become Wizards, Clerics, or any class other than Commoner, and the kingdom would have to pay all or part of the cost associated with making that happen if they wanted the NPC to serve in their army.
PC's wouldn't have to pay those entry fees.
So what would magical warfare end up looking like in a world governed by the Pathfinder rules?
The armies *would* tend towards the small squad arrangements that people keep mentioning in this thread. They would consist of a small group of PC's with complementary skill-sets sent on specific missions.
You and your adventuring party are already the final stage of evolution for warfare on Golarion! Sending PCs on quests is always going to be cheaper than sending expensive NPC's to get the task done.