Various Accommodation in Italy would widely depend on where you stay as Italy is quite different to other European countries such as France. (Many tourists would notably just visit Paris). This is in contrast to Italy as many regional cities are quite popular to tourists. So when selecting a city in Italy it would quite depend on when you go and where.
For example in Italy "Ferragosto" when Catholics celebrate August 15th as the day the Virgin Mary rose up to Heaven, is a public holiday and prices that week or weekend will be abnormally high in parts such as Rome. Rome peak season tends to be September, while in the summer parts of Italy it usually is in August.
The locations vary in prices as they are targeted to different types of tourists. From my experience is the following:
Rome - decent prices for standard accommodation depends how far you are from the centre of Rome (Via del Corso and monuments such as the Trevi fountain) and amenities you want to pay for such as air conditioning. Popular all year.
Florence - You want to find accommodation nearest to the Duomo or Ponte Vecchio, as prices depend on how far you are from the centre of Florence, but cheaper then most other Italian cities. Popular all year
Milan - Expensive city for accommodation, prices may depend on when fashion shows are in town - Popular all year.
Venice - Expensive accommodation depending on how far you are from St Marks square. Popular all year
Amalfi Coast - which includes Sorrento and Positano - resort style accommodation and is expensive. Popular in Summer.