Storing a caravan over winter

In 2009, my wife (Ginnie) and I spent 9 months in Britain touring around a number of camping sites. We noticed at a couple of places a lot of unused caravans in a farmer's field. On asking, we were told they were being stored for the owners.

On buying a caravan ourselves, and leaving for Australia again after our holiday, we'd like to store our caravan in such a place until we come back in the following year for another period. 

Does anyone have any idea of the cost of storing a caravan? . . . period from October to April.

Are some locations better for storing than others, e.g. away from the coast with its salt spray, etc.