We'densday seems to be bargain night. We spent 120 pesos for center orchestra (for 2) this week to see Chanteclare Tango. Next week we're seeing Wicked in NYC, $358 USD for far lesser tickets.
As Tomataiki says, go to Ticket Buenos Aires as like Times Square in Buenos Aires
It is a steps from Obelisco on Diagonal Norte and Cerrito, Astor Piazzolla Plaza.
On Wedensday to Sunday, from 11am to 8pm, you can choose from their offers
and buy some donation ticket for the Theatres Association, in accordance with the amount of the ticket price (usually very good places) and go to the BOLETERIA (ticket office) of the theatre where you are going to see the show, show the donation ticket and you can get half price, so that not exactly half but in the end of the account you are paying around 60% of the price so it is good. Only for the show of that day you get the ticket. So, I go there to look for the lucky if the show is sold out,but maybe you can get good places and the ticket even on that day.