Ibiza Explorer: Chapter 2. - Ibiza Transport
You can get to the town from the airport by taxi or bus. If you choose to go with taxi only choose officials, even if you have to wait in a queue. The bus "10" costs 3,5€ from the airport to the town. From November to April every 30 minutes, from April to October every 20 minutes, and between July and August every 15 minutes.
The bus "9" goes to San Antonio, the cost is 4€, starts every hour. The bus "24" connect the airport with the Cala Nova, Es Canar, Santa Eularia. The cost is 4€, starts every hour. The island has 3 main bus stations, one in Ibiza Town, one in San Antonio and one in Santa Eularia.
The tickets costs vary between 1,55-4€, the ticket can't be bought from the driver, you must get them before you get on the bus in ticket offices.