Maclarencat's answer is spot-on. The value of the garden land (as per sallybb's answer) is irrelevant.
They have an asset they wish to dispose of which is potentially worthless unless the property has a legal right of access to pass/repass.
Unless you are experienced in these matters, you should be dealing through a solicitor to get this right. A solicitor would necessarily have much of a clue as to the value of the access rights. A simple answer is 'as much as you can negotiate', but in reality perhaps start at 35% of the seelling price of the property and see what happens from there.
Do not lose sight of the fact that you may have to live next to these people for some time to come and they might become difficult neighbours. Depends how thick-skinned you are to such matters.