Why are they so expensive ? I've no idea but all vehicles are in Spain but it does vary from region to region. 4x4's tend to maintain there price well ,as you say,regardless of the mileage. Personally , unless you are mechanically proficient, I wouldn't touch a 2nd hand spanish 4x4 unless I knew who had owned it from new. Depending on how old a vehicle you wish to buy it might be better looking at new ones ,or even a Santana , as that is more or less the same , although there is no tie up with LR anymore.
When I was looking 3 years ago I found that the farther I looked away from the south of spain, the cheaper they got. Madrid /Barcelona had the best cared for & cheapest 4x4's.
I raed somewhere a while back that the average spanish mileage is around 35,000kms a year !! I thought it was nonsense at the time but assessing it with my own kms & that of others , It's probably spoton. My neighbour , who is now retired & appears to go nowhere , averages 800kms a week. I know this as occcasionally I borrow the vehicle. I average 500kms going nowhere !!