I have been to many shows by just going in to the box office and asking if there are any bargains.
However it has only been one if me (not at a weekend) and I am happy to see anything, I just walk round the west end looking. I only went specifically to billy Elliott as that was off the beaten track as it were.
I never go to the kiosk in Leicester square that seems expensive.
Usually pay less than £20 sometimes ascheap as £10 but I haven't been for a while!