I would say it depends on the brand you're looking for; a while ago I found Pucci t-shirts at the Mall (for the better half) that while not cheap on their own were far cheaper than anything that was available at the Pucci store in Milan (via Montenapoleone).

So if you want something from Corso Buenos Aires, there are other discount malls that you can shop as well as the "saldi" in January and July. If you want something from Via Montenapoleone, the Mall is your best bet.