How much does Sky TV cost in Spain?

The same monthly charge as it does in the UK!
If you are offered Sky TV in SPain for anything less than the real monthly subscription, they start from £20 per month, then you are not paying Sky TV directly, and are not getting an offical and "legal" Sky viewing card or subscription.

If your installer says you must have your satellite receive connected to the phoen line or ADSL router to receive SKy TV, then this is untrue, and more than likely an illegal card sharing system.