For that number, often hiring a villa for the duration can be cheaper than booking say a hotel or 2 apartments type thing, there are heaps of holiday villa websites on the internet, you could book that and book a cheap flight seperate, I know Easjet fly to several Spanish airports now. Otherwise travel agents tend to do good group deals now (not as expensive as they used to be as they have lost of a heck of a lot of business through online sales) or try some of the cheap operators like Thompson or Cosmos. Another idea is holiday villages, like Haven type thing but over in Spain, Eurocamp and such like. I'm sure you'll find something suitable, just keep doing the research.