I don't know about making armies more expensive, but you can certainly reduce the amount of money flowing around. If you go to the campaign file (located in something like data,maps,world, ,can't remember if this is the exact location) called Imperial Campaign, you can edit things like starting money for all factions, but more importantly, the value of trade goods.

If you reduce the value of trade, that will significantly reduce the amount of money available, and so limit armies. I'm sure there are other ways, this is just one.

And finally, as always, create a back-up of any files you edit in case things go wrong.

edit: typo