Went for £9,000
I just had a look at the web site. Tickets vary between £10 and about £30 depending on where you sit and how popular they think the show is likely to be. If we suppose they put you in the cheapest seats, you'd need to see 900 performances to get your money back. Over the course of a lifetime, that is doable, although you'd have to see some shows more than once.
When I made my original comment, I was thinking of £1,500 and a ticket price of £30, which comes out to 50 performances, which is five times a year over ten years.