Land prices can vary drastically due to factors such as the availability of present or future planning permission or development and the extent to which local farmers are motivated to outbid each other for a holding that might be strategically or sentimentally valuable to them.
Auctioneers would be the best source of information on specific purchase opportunties but be careful as they will be working in the vendors' interests and certainly not in your own. Unless you have a very specific idea what you are looking for, and a very detailed knowledge of the local market, you stand a strong chance of being ripped off.