I see that Mushroom25 has been to the Aena website ahead of me :-)
For the random dates I've tried in August, Shuttle Direct quote single fares of �10.48 per person for shared transport or �12.35 per person for private transport:
http://www.shuttledirect.com
Resort Hoppa quote a RETURN fare of 30.50 euros per person for shared transport. (That equates to single fares which are very close to those offered by Shuttle Direct). The return fare for your whole group is quoted as 316 euros. (Once again, calculating equivalent single fares, per person, gives a figure close to those offered by Shuttle Direct):
The cheapest I've found, however, is a property company who also offer airport transfers. (It seems that the service is not restricted to clients of their principal business). They have an 8-seater minibus and quote 75 euros for a single journey. That works out at about �6.50 per person. (Note that this webpage won't load in Firefox. It works fine with Internet Explorer):
http://www.dummerjones.co.uk/collections.htm
Hoping something here helps,
Chris