Check all the hire companies out - I used to use BCO (Booking Centre Online) but found that their attitude stinks - and I got a cheaper car by going to Sol-Mar direct.
There's an Android app called Gas Stations Spain - Hermanos Navarro (the one in SP) is listed in there.
It'll tell you the cheapest near you.
Remember - if you're in a UK registered car and have no tax disc - and you're using Spanish roads permanently - you're avoiding paying Spanish road tax.