If you book through the likes of Maxy Travel in Fuengirola, the going rate for the Gib trip is 12.90 euros.
http://www.maxytravel.com/

no good! its in spanish.

Hi Les. On the Spanish site, just click on excursiones, where you want to go to, then precios (prices) is all you need.
Another site doing trips to Gibraltar is CostAfrica.
http://www.costafrica.com/
The home site is also Spanish, but click on the union flag and you'll soon make sense of it. Unlike Maxy, however, I do not think it gives prices.
Maxy's 12.90 euros for Gib is about average, and I suspect the travel shops in Calahonda will match it.