NOTE I'M MAKING THESE FIGURES UP!!!!
Say that land you can build on goes at £100,00 per acre
Land used for arable farming goes at £15,000 per acre
Land used for cattle farming goes for £5,000 per acre
Land used for "convenience" goes for £1,000 per acre
Pick which you;re gonig to use it for. If it's in the first 3 you should check local estate / land agents and work out how much you would pay per acre. I suspect it'll be in the latter catgory, in which case all you can do is work out how much you would be prepared to pay to make your garden nicer. I'd certainly be starting in 3 figures not 4, unless you're buying a massive field