It depends on how you get on with your neighbour and do you have mortgages.
My neighbour owned a piece of land next to my house and i asked if i could buy it, he said we could do what we wanted with it but I felt if he died or moved the next neighbour may not feel the same. We both had mortgages with the Nationwide so we contacted them to see what we do, they sent a surveyor and he said if my neighbour lost the land it wouldnt make any difference. If I gained the land it would put �5000 on my house as I could park or build on it. The Nationwide said he should sell it to me for �1000 minimum or �5000 max. My neighbour said �1000 and I pay the legal fees which worked out to about �800 then we sat in the garden and drank a few bottles of wine.
I think the price depends on how you get on with your neighbour, but if you have mortgages the land belong to the banks so they say what its worth.