Another way to reach Cagliari is by ferry. There is an overnight ferry service from the Italian mainland (Civitavecchia or Napels) directly to Cagliari.
Other ferry lines, also from France (Marseilles, Toulon, Nice), have Porto Torres or Olbia in the north of Sardegna as destination. From these cities you can cross the Island by car to Cagliari (about 250-300 km).
In summer time advanced reservation is necessary.
The direct ferry connection to Cagliari is handled by the Tirrenia Company (www.tirrenia.it). A single trip for 4 people + car can easily cost about 300 EUR (without sleeping cabin) to 400 EUR (with sleeping cabin).