If its for agricultural use only, the quality of the land, how well it was maintained by the previous owner, sheds and animal handling facilities, water supply, quality of fencing, etc are all factors. Generally the smaller the plot the more expensive as there would be more potential buyers. Almost all land has site potential as agricultural land owners have a better chance of getting planning permission for a one off house than a site owner, so road frontage, distance from towns, availability of servives such as water, ESB, sewerage would be an influence. The main factor is the amount of others interested, as in "The Field"!