I don't speak for BALPA; I am just a member. As far as I know, BALPA charges so much because (I believe) it is required by its own rules to recover the costs incurred by the event. Ring up the hotel and see what the hire of the facilities costs - don't forget the IT provision, or lunch. All the attendees from the airlines will be paid a contribution towards their expenses as well. On top of that is the secretarial and organisational time put in before the event, as well as on the day. None of this is cheap. Making money is not the aim - if BALPA wanted to do that, it could do so much more easily!
As a full member, I pay BALPA around £800 a year for my membership. £55 is one of their less expensive offerings!