The Royal Opera House is not usually the place to find a cut-price deal, but they've made an exception with a couple of special Carmen showings. For the performances on 18 and 24 June, all seats are reduced to £37.50 or less. On the top-price seats, this represents a discount of £150+ (!). Best value seats remaining would appear to be mid/rear amphitheatre (£15) and balcony boxes (£105 for 4).
There is a catch - the shows are being filmed for the first-ever 3D opera movie, and the position and size of the special cameras may restrict views to some extent.
It's also not the greatest production or the most star-spangled cast. But it is cheap.