Try.
doyouspain (Website)
This will give you lots of variables.
Other option is to book two in tandam with the drop off and pick up around the same time.
Same companies or different companies?
Otherwise email a few rental firms, they may offer you a deal, but generaly the above website is cheaper than booking direct with hire companies.
Go with the companies you know and trust, which are not always the cheapest!
You can also take out your own annual car hire excess with icarinsurance (website) (Have a read) that will also save you some money, instead of paying excess at desk.
Hope that is of some help.
PS: I dont work, or have anything to do with the companies mentioned.
Good luck!
Let us all know how you get on.
Paul S