I had a similar experience last year: a professional astronomy conference in my neighbourhood (a rare event) but with prohibitively high entry charges. I'd like to see some provision, indeed encouragement, for interested members of the public to attend these events as spectators, preferably free.
In my experience of conferences, no-one will notice if you haven't paid the fees; just wander in and sit quietly at the back. Of course if you intend to do it properly (schmoozing, asking incisive questions, eating the sandwiches, etc.), you should pay!