I've repeatedly been getting error messages when trying to get a quote from the Trasmediterranea website. However, the ferry crossing probably won't be cheap as it typically takes around 33 hours to get from Cadiz to Tenerife. (The Canary Islands are culturally and politically European but geographically they're an African destination). Perhaps you'll have more luck?
http://www.trasmediterranea.es/trasmeweb/inici o.do
If you're starting from, say, London remember to add on the cost of the cross-channel ferry, the cost of fuel (�300 each way in a moderate sized campervan?) for the 1400 miles driving you've got to do, just to get you to Cadiz, and nearly �50 in motorway tolls. It would almost certainly cost you well over �1000 for the complete return journey. With no time for sight-seeing en route, it will take you at least 5 days to get to any of the Canary Islands from the UK.
Chris