I'm assuming ambush has a value, so making a 1 or 2 drop with ambush would imply that immaterial either has no value or has negative value.
Ambush has a value that is relative to the average number of ambushers on the field per turn.
Material creatures can be fractal'ed thus increasing the average number of ambushers. The amount of change depends on the casting cost of the creature. Higher casting cost creatures have smaller fractal'ed armies than cheaper ones.
Immaterial creatures cannot be fractal'ed thus decreasing the average number of ambushers.
Immaterial has a value dependent on the threat value of the creature it is on.
End result
Threat Value: 1 (attack) +1 (Ambush[Immaterial]) =2
Ideal Casting Cost: 1 (attack) +0 (HP value) +2(Immortal[2]) +1 (Ambush[Immaterial]) -1 (Aether bonus)=3