Here's a great thread from Linda Bloomfield (from 2018 so some bits might be different now) on "how to see theatre for cheap/free".
https://twitter.com/LindaBxx/status/1062777403116867584
Some of these membership clubs (some are mentioned in her thread) also offer cinema and other entertainment tickets). As far as I'm aware these are London-focused (I don't know about other cities I'm afraid).
• The Audience Club
• Central Tickets
• ShowFilmFirst
• My Box Office
• Play By Play
These can be seat-filler tickets ('papering' where a new production needs a bit of a starter motor to get the word out) or previews to get market research feedback. Some are free to join, some require that you use a minimum number of tickets to maintain membership.
All require discretion and not blabbing about how you got your tickets (because if you got a freebie the person sitting next to you who paid full price might be a bit miffed if they find out).
Articles about the clubs
• The Guardian
• Money Saving Expert
• Sunday Times (£)
• Broke in London
• Ian Visits
I also wrote a blog post in 2016 about some ways of getting hold of tickets for things that have sold out (nothing too sinister, just being in touch with the box office, looking out for competitions etc).
Of course there are also costs in getting to and from the venue, meal out etc.
Jo