Unit cost and quality.

and champions come in groups of 4 per week, dealing more damage from a distance and moving far.

Increase the Vampire's health to 90, and the champion won't have a chance in a 1-on-1 fight. I would say that the champion is not a high-quality level 4 unit; it should be level 3.
But the champion of Dungeon, with its regeneration and terror, is a high-quality unit at an optimal price.