Have a wonderful trip. I agree the open air markets are the best places to find good prices, barter, barter, barter.
I was there in 2001 and again in 2003. The euro has made a big difference in what you get for your dollar.
Florence has lots of outdoor markets selling leather, I think most of it comes from Turkey. If you want Italian Leather expect to pay. I could purchase lots of leather items for the same prices I could here in the USA. Now if you want a Gucci or Prada bag, you need to shop in the specialty shops, there are fakes everywhere in Italy, so if you want the real thing you need to shop in these shops. Example: Prada and Gucci Bags start around $400.00, you get your duty free tax back. When you subtract the tax your getting the bag for $300 and something. Good deal but at these prices I didn't take back as much as I'd like.
I did find some good prices on jewerly on the back streets in little shops, but you work to find them. You need more then a couple of days to find deals.
As far as Venice goes everything was expensive, very few deals here. The Murano glass is beautiful, but again expect to pay for the real thing. As far as the mask go, mass produced somewhere. Again if you want the real thing find a little shop where there painting them. Yes you'll pay more.
Found no deals in Rome. Lots of venders with there hobo sheets set-up on the road to the Vatican, they are selling repro's that are pretty good.
It comes down to if you don't mind getting repro's there are great deals, but if you want the real thing you will have to pay, not to many deals here.