It’s really hard to compare car hire costs in Spain because none of them are honest about how you pay for fuel or what insurance costs.
A lot of them make you pay for a full tank of fuel at an extortionate rate, which is OK if you are touring, but if you’re not travelling far, is a rip off as you will just give most of the fuel back to the hire car company, and they will resell it to the next customer.
They also say that it’s fully insured with no excess, but then when you get there they say its X Euros per day to cover the tyres/glass/underside of the car, which also adds loads to the price. Alternative is to leave a massive deposit on your credit card, which may be inconvenient.
The worst at the above are Record Rentacar and Goldcar. Avoid these at all costs. They are also at the airport and generally have very long queues to pick up the car.
We have used Automotive via www.carjet.co.uk successfully and paid Carjet’s excess insurance at £2 per day, which is much cheaper than what you will be charged on the day. Never had any problems with being charged extra later, and cars have always been fine. I think we paid for quarter or half a tank of fuel, which is much better than a full tank.
Lots more advice here on Moneysaving Expert.
Depending on the car size you need, I would say it will cost £10-20 per day for a small to medium car, but might be more in high season (we always go outside the main summer season).
Have lovely hols and good luck!