I would say 1000 euro an acre is very realistic. Four years ago I had figured out the same price and could have bought many finca's in that pricerange. Prices haven't really going up since ;-) Of course there is no 'house' included in this price, just a ruin.

The main problems you will encounter will be with building something on such land. It is not impossible but you will have to consider if you have the time, money and spirit to go this way. The easy road is to pay some more for a finca with a legal building already on it.

Good luck, Mark