If you fly into Bola Airport, you can get to the city center and bus station by cab. The standard fare is 20 HRK for boarding and 10 HRK for a 1km journey. Thus, a cab ride from Brac Airport to the center of Bol will cost about 160-180 HRK.
From the pier or bus station in Bol, you can get to any other town on the island. Ferry and bus tickets cost from 28 HRK (depending on the distance).
If your plane lands in Split, the route to Brac will be a bit more complicated. First, you need to come from the airport directly to Split, to the marina. There are two options: book a cab (from 250 HRK) or take a bus (28 HRK) to the Trajektna Luka stop and walk about 200 meters. Then take a fast catamaran to Bol or Supetar ferry. During the holiday season, the first ferry leaves at 5:15 and the last one at 23:59. The boat ride will take approximately 50 minutes.