Cars in Spain are EXPENSIVE. I would say 3 times more so than the UK for the same car with a lot more mileage on. If you are here short term much better to drive your own car over, however if here long term then you would have to pay for conversion over to Spanish plates which can cost 1000 e on an average car (my friend was recently quoted over 4000 euros to change his over and now has to drive it back home and sell it!)
Hope that helps a little.