For me, there is no other better place in Italy to squander money than in Turin. This city is meant for serious shopping. From international luxury boutiques (e.g. Hermes) to independent small retailers to family-owned leather shops, Turin offers excellent shopping opportunities for those who know what they want.
This being one of the most important cities in Italy, needless to say all the major players in Italy's fashion industry are present. On Piazza San Carlo and the adjacent Via Roma, as well as Via Giuseppe Luigi Lagrange parallel to Piazza San Carlo, one can find international boutiques and prestigeous names such as Hermes, Max Mara, Ferragamo, Mont Blanc, Louis Vuitton, Armani, Gucci, Prada, Chanel, Dior, etc., as well as many other Italian brands. The advantage of shopping in Turin is, being somewhat off-the-beaten-tourist-track, you can look around the shops at your own pace, nobody will try to hassel you, coerce you into buying something, or attempt some aggressive selling tactics on you. And you don't have to push you way around the boutiques amid masses of package-tour tourists from the US and Asia, nor fight your way through the shopfloors till you reach the cashiers. Unlike Florence or Venice, shops in Turin in general do not carry much tourist stuffs. Which is good as I am sick of seeing precious shop spaces in prime locations stocked up with cheap Made in China tourist souvenir and memorabilia.