The fare is the same in both directions: about €23 to €25 one way on the direct crossings, from €15 on the slower Krilo boat via Milna. Buy online rather than at the small Hvar ticket window, which has a queue and a cash-only reputation in high season.
Book the morning departures early. Everyone leaving the island on a Saturday in August wants the same two boats, and they sell out days ahead.
If you are connecting to a flight, do not take the last possible catamaran. A cancelled or delayed boat leaves you with no scheduled option, and Split airport is another 40 minutes from the port.