I have found returns online vary greatly, via the italian equivilent of national rail enquiries I found tickets for between 55 and 75 punds for the standard or flexi options.
If the amount of time to travel doesn't put you off then I owuld phone the local tourist information office and get them to sort you out.
I hope I am not eaching you to suck eggs, but unless you are planning to travel to Rome for a specific event, it really cannot be done in a day. Bologna is a stunning town and only an hour away. Of course you have the lovely venice near by and Florence and Sienna make a good day trip.