You won't find cheap clothing retail in Italy. Believe it or not the cheapest clothing is in the US. I save up every year and take an empty suitcase with me for my trip to NYC and go beserk.
Having said that there is a designer outlet mall outside of Florence. I have heard of it but never visited. There are day tours to it. From the Viator website : You visit Prada first and then go onto the mall. "At the Mall you'll have two hours to shop for Agnona, Alexander McQueen, Armani Jeans, Balenciaga, Bottega Veneta, Burberry, Emanuel Ungaro, Ermenegildo Zegna, Fendi, Giorgio Armani, Gucci, Hogan, I Pinco Pallino, La Perla, Loro Piana, Marni, Pucci, Salvatore Ferragamo, Sergio Rossi, Stella McCartney, Tod's, Valentino, Yohji Yamamoto and Yves Saint Laurent." Have heard very mixed reports some say fantastic some say not worth the waste of a day.