There supposedly are less expensive seats (theatremonkey says Row BB is 39.50)- there have been reports of people being upgraded from the circle to the stalls - and there is at least one "secret" discount website which has tickets for, I think, a membership fee. I read about it on theatreforum.com, but those posts, I think, were deleted by the moderators. However, a websearch may locate it.
I paid £24.50 for a seat at the back of the Grand Circle two weeks ago, then when I arrived at the theatre I was told the GC was closed and I was upgraded to a £67.50 Dress Circle seat. I'm addition, my friend arrived at the box office around 5pm on Tuesday last week and asked if they had any offers and they sold her a seat in the Row D of the Stalls for £32.50.