Unlike a kidney, Breath is safely re-transferable. So, if you give your own personal Breath to a Drab and they give it back to you, you've lost nothing. On the other end of the scale, it's not like people selling their hair. So, somewhere in between. I don't think we have enough information yet so place it along that scale with much precision... yet.
That I remember, I'm not quite sure just how much of a difference being a Drab makes to one's daily life. Or lifespan. Certainly with a large amount of Breath you effectively become immortal, but I don't remember any indications that becoming Drab halves your lifespan or the like. We see Vivi go cold turkey as it were, but she was going from hundreds to zero, not 1 to zero, so I'm not sure just how big a difference it is. Certainly lots of people seem to find worthwhile to sell their Breath, so how bad is it?
On a side note, you talk about Awakening being immoral, but what you're mostly talking about is collecting / selling / transferring Breath - I find this the most interesting aspect of the magic system - basically allows a bunch of nobodies to pool together their Breath and do some damage, providing their 'avatar' can be trusted and has any skill of course. At an economic level, you could 'pawn' your Breath, use the money to start up a business, make a profit and then buy your Breath back (or rather someone's Breath). I also wonder, do grandparents 'will' their Breath to the younger generation just before they die? You could stack up Breath down through the generations. I imagine that when you're nearly dead, your Breath would be quite weak, but it could still be worth it, particularly in the long term.
On another side note: one thing brought up several times is that it's not possible to have Breath ripped from you in practice. You have to willingly give it away, either by speaking the words or using strong enough mental thoughts. But, is this really true? After all, Nightblood seems to 'eat' Breath which doesn't seem that dissimilar from ripping Breath from someone involuntarily. Maybe there's an implied 'contract' when Nightblood is held / wielded...?