If you're a BFI member, it's typically £17 a ticket (£20 I think for the bigger films/Southbank Centre/gala performances).The Lite passes look ok, though. I missed the London one this year, but last year I saw three films. Cost me more than 50 quid.
I like seeing films that might not otherwise get a release so i bite the bullet. (And the q&as I've seen this year have been better than usual too.)