Heroes 5.5 WarGame Edition - Tactical Edition.

I disagree. Imperial Knights should have very strong training.

Why? They are just ordinary medieval knights, not heroes.
Of course, you can explain anything, but it's easier to rearrange the tiers so that it makes the most sense – a knight is clearly not on par with an Ent, a Balrog, or a Hydra.
Especially the mass-produced knight, who undergoes mass training.

Oh... I'm a little sad... will there be heroes in the world who will defeat a dragon in an unequal battle!

Heroes are separate, units are separate

Specifically regarding the price and strength of human units:
150 Militia (or rather, a pair – thanks to the Reserve ability = duplication)
250 Archer
400 Swordsman
600 Monk
1000 Knight
1500 Griffin
3000 Angel

For comparison:
250 Assassin
400 Fury
600 Minotaur
750 Predator
1000 Witch
3000 Hydra
6000 Dragon

Moving the knights to tier-5 (and the monks to tier-4) is not only a matter of taste, but also of balance. It allows for a much better balance of retraining, as the difference between a monk and a knight, as well as a swordsman and a monk, is significantly reduced.