hehe, yea, I make a lot of monks. The cost and time of creating monks is the price paid for the added abilities of paladin & crusader, as you'll notice the other costs are the same as horseman & guard.
I might give the monks more mana though because they seem to run out a little too quickly in big battles. On the flip side, ever tried to kill a paladin that's getting healed by 5 monks? You basically have to run them out of mana or kill the monks 1st. But still (pre-priestess) in a large scale battle between corruption & enlightenment with approximately equal forces, I've found corruption to be more powerful each time. I have a strong feeling that earthquakes will help even this out, but they aren't finished yet. One of the main things earthquakes are supposed to do is to (randomly) knock units down, rendering them temporarily unable to attack, until they get up again. Since the priestess' earthquake is using impact damage, it should effect Lichs & skeletons pretty well.
Also, the priestess has a unique type of armor that's a kind of new test to the game. It's called "divine" and it's basically highly resistant to physical attacks, moderately resistant to cold & fire attacks, but vulnerable to energy attacks. She doesn't actually wear armor, it's more a state of her being, faith, and relationship to the ethereal that these attacks don't effect others more closely tied to the physical plane.
EDIT: Oh, one more note on the piety thing. I actually make lots of guards & archers as well, which don't use piety. Guards are just about the slowest units on the tech side, but they are also the most heavily armored (I think paladin has as much armor now). Archers are just deadly. The only things they sucks at attacking are liches & skeletons, but they take down most others pretty quickly. Not to say that it all couldn't use more balancing to find that nice little rock, paper scissors paradigm