Because with no constant we can say any ability costs any cost we want it to? Immortal and Phase Dragon both cost immaterial at 2 and their threat density is almost as far apart on the spectrum as you can get. Why would ambush change this?
The value of Immortal depends on threat value (Attack + Offensive Skill Value). Creatures that are more threatening would be targeted by more control than creatures that are less threatening. Based on Immortal and Phase Dragon I theorize that the value of Immortal is equal to1 plus half the threat value (rounded up).
Immortal
Static Value = 4 (attack) +0 (0<HP<6) -1 (Aether Bonus) = 3
Variable Value = 3 (1 + 4 x 1/2) = 3
Balanced Casting Cost = 3 (Static Value) +3 (Variable Value) = 6
Elite Immortal
Static Value = 5 (attack) +0 (0<HP<6) -1 (Aether Bonus) = 4
Variable Value = 4 (1 + 5 x 1/2) = 4
Balanced Casting Cost = 4 (Static Value) +4 (Variable Value) -1 (Upgrade) = 7
Phase Dragon
Static Value = 8 (attack) +1 (5<HP<8 ) -1 (Aether Bonus) = 8
Variable Value = 5 (1 + 8 x 1/2) = 5
Balanced Casting Cost = 8 (Static Value) +5 (Variable Value) = 13
Elite Phase Dragon
Static Value = 10 (attack) +1 (5<HP<8 ) -1 (Aether Bonus) = 10
Variable Value = 6 (1 + 10 x 1/2) = 6
Balanced Casting Cost = 10 (Static Value) +6 (Variable Value) -2 (Upgrade) = 14
I think you may find that Phase Dragon and Immortal do not agree on a constant value for immaterial but rather for a variable value dependent on threat value (attack in their case)