In short
The Poetto, eight kilometres of sand, is twenty minutes away by bus from the centre. Few regional capitals offer that.
The old town, the Castello, sits on a fortified hill above the gulf. You go up by a free public lift rather than the ramps.
What it really costs
Cagliari costs less than northern Sardinia, where the Costa Smeralda pulls every price up. Over a month the difference is twofold.
The island is poorly served by public transport: the train is slow and infrequent, and exploring Sardinia from Cagliari means a car, to be added to the budget.
What nobody tells you
In August the whole island is booked out and ferry crossings are reserved months ahead, vehicles included. Flying from the mainland is often cheaper than the boat.
Flamingos live in the lagoons on the edge of the city, all year round. You see them from the road to the Poetto, with no excursion.
Many shops close for several hours in the early afternoon, in season too. Errands are done in the morning.