You can find the information here:-
At STEP2 of our online booking process, click on a few different suppliers as they all offer different amounts of deposit. Next to each car photo is a link entitled IMPORTANT INFORMATION, click on this and then on DEPOSIT, to view the size of deposit.
For example, CENTAURO 1050€, INTERRENT 300€, RECORD 950€, NIZA 1110€, FIREFLY 1110€
Some companies ask for a very high security deposit, which is held against the credit card until the car is returned. They do this, so you will be persuaded to take out the extra insurance at the desk to reduce the deposit.
It is normal for a deposit of around 300€
I hope this information is helpful although this refers to Spain but it is much the same in Italy.