What would you change in Heroes of Might and Magic IV?

In Heroes 4, many prices are not balanced...

Yes, that's a drawback. It's a pity that they didn't make the Titan equal to the Black Dragon in health, as it was in Heroes 3, and not to the Hydra as in Heroes 4, because Chaos has an advantage, but the price of Black Dragons is a balm for the soul of Haven mages.