Officine Sbiellate is a bar located in the Porta Lodovica neighborhood, pretty close to the Bocconi University. Due to its proximity to the university, it's usually attended by university students and prices are very much affordable.
It mostly has outdoor seating, so it's great for warmer months. If you go on a Tuesday from 9 pm to 12 am, they sell beer for 3€, a bargain for Milan's standards!