Is it cheaper to fly to Croatia from Rome or go by ferry?
There are at least a dozen ferry routes between Italy & Croatia, but only from Italy's Adriatic (east) coast. There are no ferries from Rome to Croatia. The east coast ferry port closest to Rome would be Pescara.
You can get a Europebyair flight pass from Rome to Split for $99.
http://www.europebyair.com/efp/index.jsp