In the last 7 days, the lowest price for car rental was 70€ per day in Alghero.
It is not true that rental car prices tend to be more expensive at airport locations due to the higher daily rates .. However, be sure to double check the prices and book well in advance.
It is not true that cars rented at the airport are almost always more expensive due to taxes, it is just a matter of booking. Making a reservation in advance allows you not to queue.
By booking in advance you will surely have many discounts, and will allow you to have no surprises upon your arrival.