I know this has been correctly answered already, but I want to share my answer, anyway:
Without me you'd be lost, That's the smallest cost.
Lost in space (at least, assuming it was switched off. The universe wouldn't look anything like it does if it had never worked.)
You use me every day, More than I can even say,
Even if you're in orbit.
You do but don't see me, So whatever could I be?
You see the effects of it, but you don't register it because it's so normal.
Inside, outside, all around, I'm even underground.
Even at the centre of the Earth, you're gravitationally affected by the rest of it. Even discounting that, you're affected by the gravity of the Sun.
I can make no sound, Can't really be bound.
You can be bound to it, though.
So tell me if you can see, What it is that I might be?
Ok, Gravity only works if I still didn't realise this was supposed to be a clue, too.