from my experiance with it no and it dont use that much end either, i can run assult, manauvers, tactics, hover, DF and FB and still gain end faster than i use it 
Answered the question myself - when it causes knockdown there's extra cost.
IN MEMORIUM OF GAYBABY
CRUELY TAKEN FROM THIS WORLD WHILE SO YOUNG
And as an aside, it completely rocks!....
I would get in and use it now, before the incoming huge nerf bat 
Things it doesn't push back: Carnie Strongmen, The Rularuu Brutes, some Elite Bosses and all Heros (the npc kind...not sure on player kind.. ). Virtually everything else is pushed back.
Can't remember any more, but they probably are out there!
Force bubble doesnt cause knockdown, it's only repel with and END cost that's the same no matter what it's affecting. The one thad does knockback is repulsion field, which has additional cost for each affected target.
Force Bubble DOES cause knockdown, but not with a particularly high accuracy or high magnitude.
However it's just SOOOO amusing to fly into Grendels Gulch in the Hollows, find a patch of smoking ground and toggle it on and watch the Magmites/Pumicites go flying
Warning:
The above post may contain Cynicism, sarcasm and/or pessimism. If you object to the quantities contained, then tough.
Hmm... Can't say I've seen it do knockdown even once when using it... 
I have - anything that gets trapped inside the bubble can be knocked down. It doesn't happen that often but it does happen.
IN MEMORIUM OF GAYBABY
CRUELY TAKEN FROM THIS WORLD WHILE SO YOUNG
Does it have an additional end cost for each thing it pushes back?
IN MEMORIUM OF GAYBABY
CRUELY TAKEN FROM THIS WORLD WHILE SO YOUNG