I have rented a car twice in Barcelona. The first time was during a 4-day stay and the best price I could find was with
www.alamo.com. I picked up the car and returned it at the airport.
The second time was on a longer stay where I did not want to have the car during the whole vacation. I ended up renting a car for 3 days. The pickup was at the Sants train station and I returned it at the airport on my departure day, without a drop-off fee. This was with
www.avis.com. So for starters, I would go to the Alamo and Avis websites and check their rates.