Island Brac can be reached by ferry in two ways.

1) From Split
Line - 631 Split - Supetar (Brac) - approx 50 minutes drive
TIME TABLE: 631 Split - Supetar (Brac) - click here

prices:
approx 23 EUR for the vehicle + driver (high season) - a one-way ticket
approx 5 EUR per adult - one-way ticket

2) From Makarska
Line - 638 Makarska - Sumartin (Brac) - approx 59 minutes drive
TIME TABLE: 638 Makarska - Sumartin (Brač) - click here

prices:
approx 23 EUR for the vehicle + driver (high season) - a one-way ticket
approx 5 EUR per adult - one-way ticket

Locals recommend the first line from Split because it is more common.

In high season, ferries often run on the island Brac without sailing schedule - non stop :)