Personally, I would recommend A Streetcar Named Desire with Rachel Weisz.
They have 10 day seats (reg price but that's still pretty cheap) and then 20 standing for £7.50.
There sometimes are student rushes (A Little Night Music had it before it closed) but its more rare. Usually it's just general concessions and it's usually mostly only plays, or if it's musicals, its about £20-25 which is still $40-50 CDN.
Also, Blood Brothers has been around forever, and it'll probably be around the next time you go. See something more current that you'll never get to see again.
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