I haven't been to a Metropolitan Opera live cinema screening for quite a while. So I was shocked to find the going rate in London has risen to £30-£40 - about three times the price of a regular cinema ticket, and more than I generally pay for live opera. And yet they still sell out, often months in advance.
One alternative is provided by the 'Encore' screenings, rebroadcast a day or more after the live event. These are mostly priced at £20-ish, but a disadvantage for many is that they tend to be scheduled mid-day and mid-week.
For the true 'Live in HD' experience, the cheapest London tickets I can find are the Barnet Odeon's. OK, so it's a 30 minute tube ride from the centre of town (close to High Barnet station), but with seats at £15.50-£18 the prices make the journey worthwhile. And there are still plenty of tickets available, for everything.