I've used farocar.com twice, no issues at all
I usually go through doyouspain (they do more countries than just Spain) and take out excess insurance.
Currently doyouspain for a random 4 days in early September are £29 for a Fiesta for 4 days through goldcar with a full/full fuel policy.
Points to note:
- The best option for fuel policy is full/full as there's no charge as long as you return it full. full/empty(refund) will charge you a fee to fill up, and full/empty will charge you a crazy rate for a tank of fuel
- Excess insurance is cheaper than their own insurance but you will need to claim back the cost afterwards and also pre-authorise your credit card. I've done it dozens of times but have never had to make a claim
- Note all damage and take very clear photos. If the car park is dark then move the car into the light to full inspect it and also use a torch to check the chassis / roof / windscreen. Note any discrepancies or kit missing
I've had examples where hiring a car for a week has cost <£10 because they assume they will make the money on the insurance, fuel etc.