Hi Mug,
We have been looking into this very recently as we will be moving to the Murcia area in July. The total cost we have come up with including 4 days self drive van hire, tolls, diesel, two overnight stops in motels and Eurotunnel we have estimated about £1200.00
We have allowed about £400.00 worth of fuel for a long wheel base sprinter van. We have found a van hire company based in Salisbury which do one way hire, from UK to Spain (Murcia area) or Spain to UK. If you are driving your own vehicle you can knock off about £500.00 from the £1200.00 and obviously fuel costs may vary.
The best route we have been advised is Poole to Santander on an overnight ferry crossing and then about a 10 hour drive. (We are not doing it this way cos my Mrs gets ferry sick.....arrggghhh!!).
We think overall the total costing more or less balances itself out.
If you need any more info, you can send me a PM.
D.