Some quick calculations
I work for a small non-public transport operator and we have BALPA recognition. In the past 4 years I have had a total pay rise of around 24%. This is considerably more than those non-aircrew departments within our company (and this includes skilled professionals). The total payrise is 5 times as much cash as I've paid to BALPA in fees over the years.
I'm no mathematician, but it seems like a good return and much better than if I was negotiating my own payrise. Of course, all those non-BALPA colleagues also benefit...'I'm alright jack' springs to mind!