There is a thread somewhere on this site all about boreholes,probably started by me........We are looking into to it  (the old ones are the best) There is a firm in Amettla called Aragonesa de sandeos y fluidos and we plan to visit them early next month.If this thread is still hot I'll let you know.
What I do know is that it is not cheap but would put value on the property but weigh that up against the cost of having water delivered over say a twenty year period.The other cost factor which we initially overlooked was the cost of a pump and the electricity it uses to draw the water from a deep well.
I'd be interested if anyone has any actual figures on this.