hi Sophie ...
here is a more up to date information: rail.cc/en/ferry-italy-croatia/f2338
it might be EUR 25 - depends on the season.
but in most cases concerning traveling by ferry you will have to add port tax and petrol supplement.
so it will be something around 35-40 EUR.
I am not sure about fares at the day of departure as I am more a rail specialist - the ferry information is only a bonus.