Yes. As said you raise 50 HP worth of demons per Pit Lord. But the number you raise can not be greater than the number of troops in the dead stack and the total HP of the raised demons can not be greater than the total HP the dead stack originally had.
The "cost" of one demon has nothing to do with the base health of the dead stack's unit type.
____________
if i can make a noob remark to the last pic posted.
Sometimes if its possible, its actually wiser to try and fight certain mobs with only part your army instead of bringing it full.
Like in your pick....you fight a horde of dendroids and you bring as much army as to have them positioned in only one stack....wich is bad.
The weaker they think you are, the more stacks they will be split in and thus the easier it will be to grind down those stacks without taking losses.
meeting one of these single stacks mobs can soon turn into a disater with a high morale on their part.