Lots of cheap seats for London shows in the annual Get Into Theatre offer.
Click on the red banner "View by Price" from the main page, that gives a good overview broken down into Plays / Musicals / Opera etc...
And yes even big shows like Wicked are discounting for some performances, for midweek matinees it is just £25 for top price seat or £15 if you are aged between 16 - 25..