If you are more interested in just being inside the theatre than seeing a specific show, there's a great deal on Mondays. You stand in line at the theatre box office at 4pm on Mondays and you can buy tickets for 2 pesos for a show at 6pm that night. From what I understand, this is usually some type of musical recital. When I went it was a piano, flute and cello and it was excellent. It only lasted about an hour but I got to sit in a box and take in the atmosphere of the theatre (which is beautiful inside) and hear the music, all for 2 pesos which is 66 cents!