IMO, I think the unit prices are decently balanced. Having multiple wizards will make your army OP. I completed Act 2-9 & 2-10 using a combination of farmers, archers, arbalists, cock tanks, mines, and barrels. Since farmers have the shortest cooldown, place them in front of witches when they spawn to absorb their Charm spells while your archers and arbalists shoot them from the rear of your formation.