All the 4 mobile operators (TIM, Vodafone, Wind and 3) offers pre-paid SIM card with data plans. Compared to other countries, Italy is quite permissive and you don't need to provide any proof of residence to purchase a pre-paid card.
3 (Tre) is generally the most cheap operator when it comes to data plans. The downside is that the signal can be quite limited if you travel outside cities. The plan normally covers only 3G traffic, thus if there is no 3G signal you will be charged for a roaming fee.
Solutions and prices change depending if you prefer a data-only SIM (that means you will use the iPhone as a modem, but you will not be able to call or receive calls) or if you want a normal SIM with a data plan attached.
If you want a data only SIM, then the options are:
If you want a normal SIM card with a data plan, then the price should be:
Keep in mind that TIM, Vodafone and Wind tend to change their plans quite frequently. You may want to ask for some recommendation once you arrive in Italy.
Moreover, you may be asked to pay the price of the new SIM card on top of the plan price.
I don't have a lot of experience with Vodafone or Wind. Moreover, you need to be very careful because some operators require an extra charge if you use the iPhone tethering feature (as far as I remember, Vodafone was used to charge an extra price of 4€/day for it).
I'm quite happy with TIM. It's a little bit pricey, but it works well if you travel. If you are not planning to travel a lot and if you will stay in big cities where there is a 3G/4G coverage, then 3 is definitely cheaper and a good solution.