We were there to see "Proust ou Les Intermittences du coeur" Roland Petit. But of course we students are generally not so rich. So what can we do?
Wait in front of ticket counter from 15:00 until they start selling their tickets of the day at 18:45 then get cheap 10euro sheets.
But one benefit to have these "not very good" sheets at the palais garnier is that we can see the painting of Chagall painted (not sure) on the ceiling.