On a future trip, I plan to take the ferry from mainland Italy to Olbia
(on Sardinia), then go to Porto Cervo.

ARST and another company (Sunline) run buses between Olbia and Porto
Cervo, but neither service seems particularly frequent (e.g. 3-5 buses
per day....)

Taxis are a possibility, but I don't know the cost, is it too expensive?

Any other information? Thanks in advance!