This has been mentioned a few times in other ticket deals but looks like they've brought back this popular scheme. Go to the box office on Level 5 of the Theatres Building (under the spire) from 6pm each day (even for shows at Hamer Hall or the Bowl) and jump into the tixatsix line to purchase your tickets. At 5PM, they'll announce (not online) what the Tix at Six shows are and then you can be spontaneous and go to a show that night.
Normal ticket prices for shows vary and go from $39 to $235. All depends on the show, venue, date, and seating. A minimum of 20 seats are available every night. I'm guessing Fri/Sat there will be a smaller amount of tickets available then other nights.
As an alternative, check out Halftix which you'll be able to see prices online.
Being ignorant of all things cultured, what’s a good recommendation?