For a rough guide please my property values page, updated as at Sept 2010 but bear in mind that developers/land owners basically ask whatever price they think they can get rather than what the land is actually worth!
It also depends greatly on Location location location eg. if the land is by the sea you can double or triple the price. Does the plot have great views. Is it a solus plot with no neighbours and no chance of anyone building in front of you, again, double or triple the price.
It's worth checking prices in the windows of property developers and possible some property agents who may have some land prices to show you. This will give a rough guide but as i said, you could add 50 to 200% more, depending on the location.
