Taxi from Cagliari to Cagliari (city)
Actually it's not really necessary to take a taxi to the city center. This is located within walking distance of the port. Yet the streets are very steep in Castello, the historical center in the upper part of the city. You can find taxis at the exit of the port. Count on approximately € 10 per taxi (4 persons one way).
TIP: To avoid a steep walk, you can take a taxi to Piazza Arsenale. This is the highest point of the city center opposite the castle. From there it's only a downhill walk.