There are many spanish car hire firms and they all have extras ( Just like Ryan air). There are also many virtual car hire firms that act as booking agents for many firms
I have used E-mail-car-hire.com before and am hiring a car in Reus in 3 weeks time using Goldcar ( booked directly). Came across a UK based site called hirecarsinspain.com which seems to have a good reputation
In short this is what you are looking at for most car hire co's in spain
1) Prices quoted with ZERO CDW ( Collion Damage Waiever ) excess normally exclude undercarriage, windscreen , tyres ,clutch , keys and wing mirror damage.
2) If you don't take out optional insurance to Goldcar approx €8.50 a day they block a minimum of €600 on your credit card ( more for larger cars). They do the same if you have your using your own excess insurance policy.
3) For hires > 3 days they all have a full return empty fuel policy. You pay for a full tank locally
So a weeks hire say for a Peugeot 208 say is €108 . Locally you would pay an extra €60 for insurance and €70 - €100 for a full tank .
So overall you bill is approx between €238 and €268
Hope this helps