Other Car Rental Programs/Partners (ie. Alamo, Enterprise, Sixt) - a deal on a rental in Spain in August anyone? :)

I am looking for a rental car that can carry 4 people and a small amount of luggage (two adults, two kids and we know how to travel light). I will be flying into Seville, Spain on August 19th and can return the car there on the 30th (though I'd prefer to return the car on the 31st to Lisbon, Portugal). I can get a car for $541.xx for that time, a Peugot 307 through Europcar (reserved via AutoEurope. I am Gold with Hertz and Pres club with Avis, but that doesn't seem to help. Does anyone have any tips to get a rental for less?

Thanks!
Bb