Rule of thumb is applicable according to the market. A rule of thumb in Marbella, Barcelona. Madrid and their respective areas will be all different.
The Spanish has/always had a over inflated value of their asset’s and at the same time when some ones makes an offer which is under their valuation. They take it as a personal insult to them & their seven generations.
In view of the above & a fact that no one knows where ” Jaca” is there cannot be a much of a demand & families will pass on the properties to their off spring there cannot be much of a local market. I would make an offer of 50% of the asking price & see what happens.
I am sure in a depressed market & in an area that know one seem’s to know you are better off renting for as long as you want/can…